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The Pennsylvania State University

College of Earth and Mineral Sciences

Department of Geosciences

The Younger Dryas transition observed in lacustrine sediments from

Castor Lake, Washington

A Senior Thesis in Geosciences

by

Jesse D. Thornburg

Submitted in Partial Fulfillment

of the Requirements

for the Degree of

Bachelor of Science

May, 2006

We approve this thesis:

________________________________________ ______________

Katherine Freeman, Professor of Geosciences

Date

Thesis Advisor

________________________________________ ______________

David M. Bice, Professor of Geosciences

Date

Associate Head of the Undergraduate Program

________________________________________ ______________

Pratigya Polissar, Geosciences Post-Doctoral Researcher Date

Thesis Co-Advisor


The Younger Dryas transition observed in lacustrine sediments from

Castor Lake, Washington

by

Jesse Thornburg

ABSTRACT

Castor Lake in north central Washington offers an important site for

evaluation of the Younger Dryas abrupt climate reversal because it has a climate

not directly influenced by the North Atlantic thermohaline circulation. The

preservation of sediment allows for a study of the organic carbon, as well as carbon

and nitrogen isotopes. This data show that the lake and surrounding watershed

respond to the variable climate from 14,000 cal yr. BP to 11,000 cal yr Bp. There is

an increase in total organic material, along with greater terrestrial organic input

that corresponds with the Bolling-Allerod warming; these both drop off significantly

during the time defined with the Younger Dryas. This possible effect from the

Younger Dryas in an area not dominated by the thermohaline circulation shows the

extent of this abrupt change as more then just a regional event.

ii


Table of Contents

1. Introduction.........................................................................................................1.

2. Background.........................................................................................................3.

2.1 Climate Change........................................................................................3.

2.1.1 Younger Dryas..........................................................................3.

2.1.2 Western North America Younger Dryas Research...................5.

2.2 Climate Proxies........................................................................................6.

2.2.1 Stable Organic Carbon Isotopes.............................................6.

2.2.2 Total Nitrogen Isotopes..........................................................6.

2.2.3 Carbon – Nitrogen Ratio........................................................7.

3. Study Area ..........................................................................................................8.

4. Methods...............................................................................................................9.

4.1 Sampling Techniques...............................................................................9.

4.2 Sample Preparation and Analysis ............................................................9.

4.2.1 Bulk Elemental and Isotopic Composition ...............................9.

4.3 Chronology ............................................................................................10.

5. Results...............................................................................................................11.

5.1 Age Model .............................................................................................11.

5.2 EA-irms Results.....................................................................................11.

5.2.1 Weight Percent Carbon...........................................................11.

5.2.2 Atomic Carbon/Nitrogen Ratio...............................................12.

5.2.3 Organic Carbon Isotope Results .............................................13.

5.2.4 Total Nitrogen Isotope Results ...............................................14.

6. Discussion.........................................................................................................15.

6.1 Weight Percent Organic Carbon ............................................................15.

6.2 Atomic C/N............................................................................................18.

6.3 Organic Carbon Stable Isotopes.............................................................20.

6.4 Summary................................................................................................23.

7. Conclusion ........................................................................................................26.

Acknowledgments.................................................................................................28.

References.............................................................................................................29.

Appendix A...........................................................................................................41.

Mean Element and Isotope Data..................................................................41.

iii


List of Tables

Table 1. Radiocarbon dates from Castor Lake core samples...............................32.

Table 2. Core chronology with sample intervals and approximate age...............32.

iv


List of Figures

Figure 1.1 – Map outline of Washington showing Castor Lake as the black circle (Nelson,

2004) ...............................................................................................................33.

Figure 1.2 – Detailed satellite image of the Pacific Northwest, the study area is marked

by the thumbtack (http://earth.google.com/ Date Accessed: 5 April 2006) ...33.

Figure 1.3 – High resolution satellite image of the Castor Lake region.

(http://earth.google.com/ Date Accessed: 5 April 2006) ................................34.

Figure 1.4 – Topographic map of the study site, Castor Lake resides on a topographic

high with an approximate elevation of 591 meters above sea level.

(http://www.topozone.com/ 5 April 2006)......................................................34.

Figure 2 – Plot of age versus depth for the Castor Lake sample core, the two radiocarbon

dates are represented by the open circles with error bars ...............................35.

Figure 3 – Stable isotope data (δ 13 C OC , δ 15 N TN ), atomic C/N ratio values and weight

percent organic carbon data from the Castor Lake sample core, the Younger Dryas is

the region shaded. The two dashed lines indicate the location of the two radiocarbon

dates ................................................................................................................36.

Figure 4 – Weight percent calcium carbonate and weight percent organic carbon data

from the Castor Lake sample core, the Younger Dryas is the shaded region. The

dashed lines indicate the location of the two radiocarbon dates.....................37.

Figure 5 – Stable carbon isotope data (δ 13 C OC ) plotted against atomic C/N ratio values

indicating the typical ranges where material is observed (Meyers 1998).......38.

Figure 6 – Stable carbon isotope data (δ 13 C OC ) plotted against atomic C/N ratio values

from the Castor Lake sample core. The Younger Dryas occurred at the end of Zone

III and the entirety of Zone IV........................................................................39.

Figure 7 – Breakdown of total organic matter (TOM), calcium carbonate (CaCO 3 ), and

inorganic mineral/biogenic silica input for each zone ....................................40.

v


1. Introduction

The study of climate change is an increasingly important topic as the world’s

climate faces an uncertain future. An understanding of past climate changes, both cyclic

and abrupt, allow for a better understanding of possible future trends, in particular as

anthropogenic greenhouse gases increasingly perturb the climate system. Of these

potential climate changes, abrupt climate change is the most intriguing because of the

uncertainly involved with its onset. Abrupt climate reversals cause drastic changes in

climate systems, altering atmospheric and oceanic circulation and changing the average

global surface temperature.

This study will focus on a series of climate events that are recorded within the

sediments of Castor Lake, Washington, from 14,000 to 11,000 calendar years before

present (cal yr BP). The major changes that occurred during this interval are the Bølling-

Allerød warming, and the Younger Dryas abrupt cold reversal. The goal of investigating

the Castor Lake sediment record is to provide insight into the effects of abrupt climate

events on a closed-basin lake and watershed system.

The Bølling-Allerød warming was a period of warming after the last glacial

maximum. This warming was followed by the Younger Dryas, a cold climate reversal

that has been well documented in the North Atlantic region. This study seeks to

understand the effect of the Younger Dryas climate reversal on the west coast of North

America, a region not directly affected by North Atlantic climate. This study will look at

the impact of this abrupt climate change on the lake and watershed system, and will

assess the affect of the North Atlantic climate reversal on the west coast of the United

States.

1


2. Background

2.1 Climate Change

2.1.1 Younger Dryas

The Younger Dryas climatic event is an abrupt, cold reversal that occurred as the

world warmed from the last glacial maximum. It began approximately 12,650 cal yr BP

and terminated 11,500 cal yr BP (Martrat et al., 2003). After the last glacial maximum,

the world began to slowly warm in a period known as the Bølling-Allerød, which began

approximately 13,900 cal yr BP (Martrat et al., 2003). The Younger Dryas event was a

rapid reversal of the Bølling-Allerød warming, and a return to glacial conditions.

There has been extensive study on the Younger Dryas, but much of this work is

limited to the North Atlantic, Greenland and Western Europe where records of the

Younger Dryas are most prominent (Martrat et al., 2003; Taylor et al., 1997; Ebbesen and

Hald 2004; Wurth et al., 2004). There are several theories for the cause of this event,

including the Milankovitch cycle (Magny 1995), changes in atmospheric circulation and

a change in oceanic circulation (Clark et al., 2002). The leading hypothesis is currently a

change in the thermohaline circulation in the North Atlantic (Tarasov and Peltier 2005).

The thermohaline circulation transports warm tropical surface waters northward

to the North Atlantic where they sink as they become cold, more saline and dense. This

northward movement of heat energy with the Gulf Stream is currently responsible for the

mild climate of northwestern Europe (Ganachaud and Wunsch 2000). The onset of the

Younger Dryas is believed to have started by a partial or complete shut-down of the

thermohaline circulation forced by a large influx of freshwater into the North Atlantic

(Clark et al., 2002; Tarasov and Peltier 2005). There are two hypotheses for this

3


freshwater input, a pulse of melt water into the North Atlantic from the retreat of the

Laurentide ice sheet during the Bølling-Allerød, or a massive iceberg discharge into the

North Atlantic (McManus et al., 2004).

The Younger Dryas was believed to have stable temperatures for its duration, but

Ebbesen and Hald (2004) claim that fresh water forcing would have made the sea surface

temperatures (SST) fluctuate between glacial (~2 o C) and interglacial (~10 o C) values.

Their assertion is the period from 12,500 – 11,900 cal yr BP would have been the coldest

with minimum SST and low sedimentation rates; while the period from 11,800 – 11,500

would have experienced the unstable SST, with minimum temperatures and decreased sea

surface salinity values.

Another important fact about this event is the abruptness with which the reversal

occurred. The transition out of this period has been resolved to a period of about 40

years, occurring in three major five year intervals (Taylor et al., 1997). The beginning of

this period is not as clearly defined but also occurred in only a matter of decades (Taylor

et al., 1997).

The end of the Younger Dryas is marked by a warming of 5-10 o C, doubling of

Greenland snow accumulation, large changes in North Atlantic wind speeds, changing

atmospheric circulation and increased global methane which is indicative of expanding

wetlands (Alley 2000). This transition created a very unstable climate, with rising sea

surface temperatures, and increasing marine productivity (Ebbesen and Hald 2004;

Martrat et al., 2003).

4


2.1.2 Western North America Younger Dryas Research

The Younger Dryas event has not been well studied in western North America,

but has been documented at several west coast sites. This event has been observed in

speleothems at the Oregon Caves National Monument cave system site 42 o 05’N,

123 o 25’W (Vacco et al., 2005). The speleothems offer high-resolution climate records

with uranium-thorium (U-Th) dates and precise isotope records. The Younger Dryas

event was recorded within the δ 18 O isotope record as a negative excursion, but the signal

was more muted in the δ 13 C record.

One study has also observed changes in sea surface temperatures measured from

sediments in the northeast Pacific (Kienast and McKay, 2001). This study found a strong

correlation between the Younger Dryas event in the North Atlantic and sea surface

temperatures reconstructed using C-37 alkenones contained in the sediment core. The

magnitude of cooling (6 – 7 o C during the Younger Dryas) matched temperature change

reconstructed from the δ 18 O isotope record of the GISP2 ice core record in Greenland.

A study of the past 15,000 years of the south western region of United States was

conducted using packrat middens from the Grand Canyon (Cole and Arundel, 2005). The

middens were used to study the packrats past diet and type of plant consumed; this was

compared with the past distribution of the cold intolerant Utah agave plant, which

showed an upper elevation limit controlled mainly by minimum winter temperature. This

study found a temperature drop of 7.5 – 8.7 o C during the Younger Dryas.

While all three studies show a record of climate change during the Younger

Dryas, the signal varies between the three different sites. This research begins to

5


illustrate the effect of the Younger Dryas on regions far from the North Atlantic and that

the reach of abrupt climate changes as not just a regional phenomenon.

2.2 Climate Proxies

2.2.1 Stable Organic Carbon Isotopes

The use of organic carbon isotopes is helpful for determining sources of organic

matter, and thus flora present at the time of deposition. In vascular plants, the metabolic

pathway (C 3 , C 4 , or CAM) of the plant affects the amount of isotope discrimination

(Meyers 1993). C 3 plants have an average δ 13 C value of -28‰; while C 4 material has

heavier values of -14‰, these represent organic material that is terrestrial or in

equilibrium with atmospheric CO 2 (δ 13 C = -7‰) (Meyers and Lallier-Vergès 1999). The

δ 13 C of freshwater lacustrine algae varies depending on productivity and their use of

dissolved inorganic carbon δ 13 C. The availability of dissolved CO 2 (which is in

equilibrium with the atmosphere) also affects the δ 13 C values, with higher productivity

utilizing greater dissolved CO 2 and making the algae δ 13 C values more positive. On

average δ 13 C values of lacustrine algae are -25‰ to -30‰ (Meyers 1997).

2.2.2 Total Nitrogen Isotopes

Stable nitrogen isotopic distributions are useful because the composition of the

reservoir of inorganic nitrogen is generally different between terrestrial to aquatic

material. This can provide a record of environmental changes in which influence the

proportions of terrestrial and aquatic organic material preserved (Meyers and Lallier-

Vergès 1999). Some studies have suggested preservation of source information from

6


δ 15 N values in organic matter from the lake, and that there is minimal or no isotopic bias

from degradation and, giving a reliable record of the dominate sources (Meyers and

Lallier-Vergès 1999). However, other studies suggest that degradation can significantly

shift the δ 15 N of sedimentary nitrogen (Macko and Estep 1984; Silfer et al., 1992). We

assume here that diagenetic changes in δ 15 N values were uniform throughout the core,

such that relative changes in δ 15 N values reflect environmental processes.

2.2.3 Carbon – Nitrogen Ratio

The carbon to nitrogen ratio is a good way to identify the sources of organic

matter. This ratio distinguishes between terrestrial and aquatic organic sources. The

nitrogen content is relatively the same in terrestrial and aquatic organic matter, but the

carbon content varies between the two. Vascular terrestrial material contains lignin and

cellulose and has higher C/N values of 20 or greater; aquatic algae does not contain these,

and has lower C/N values of between 4 and 10 (Meyers 1993). This ratio not only

distinguishes between organic sources, but it becomes a valuable tool when used with

isotopic carbon values to determine the amount of mixing between the organic sources.

7


3. Study Area

The site chosen for this study was Castor Lake (48.54 o N, 119.56 o E), located in

north-central Washington, an area dominated by limestone geology (Fig.1). This site is

located along the margin of the Okanogan River. Castor Lake is a closed-basin kettle

lake with little to no surface outflow. The lake resides along a topographic high, at

approximately 591 meters above sea level, with an undeveloped catchment basin of

approximately 3.24 km 2 , a surface area of approximately 0.07 km and maximum depth of

approx. 11.5 meters (Nelson 2004).

The residence time for water in lakes similar to Castor Lake in this region range

from 0.2 to 20 years (Gibson et al., 2002), this value is dependent on whether the basin

hydrology is open or closed; given the closed basin of Castor Lake, the residence times

are likely to be the larger values (Nelson 2004). The lake has a pH of 8.5 and alkalinity

of 565 mg/L which leads to the assumption that most of the inorganic carbon is in the

form of bicarbonate (Nelson 2004).

The climate of the region is controlled by the Pacific Westerlies, influenced by the

Aleutian low-pressure and North Pacific high-pressure systems (Bryson and Hare 1973).

The Aleutian system dominates during the winter months creating a wet season, while the

North Pacific system moves in during the summer months with more arid, dry air

(Bryson and Hare 1973). Presently, the average winter temperatures (Dec. - Feb.) are

-3.6 o C, the average summer temperatures (June – Aug.) are 18.0 o C, and average

precipitation is 227 mm/y (Nelson 2004).

8


4. Methods

4.1 Sampling Techniques

In June 2003, a 450 cm sediment core was retrieved in five sections from Castor

Lake in North-Central Washington State using a square-rod coring system (Wright et al.,

1984) and stored at the University of Pittsburgh. The core was split, photographed, and

analyzed for dry density at the University of Pittsburgh. This study focuses on samples

between 12 and 54 cm from the lowest section of the core, spanning the years 12928 (+/-

51.6) to 11509 (+/- 87.6) cal. yr. B.P. This section of the core was subsampled at 1 cm

intervals for further analysis.

4.2 Sample Preparation and Analysis

4.2.1 Bulk Elemental and Isotopic Composition

Prior to elemental and isotopic analysis, dry samples were weighed, and carbonate

minerals were removed from the samples using a sodium acetate/acetic acid buffer with a

pH of 4. The samples reacted in the solution for 24 hours, were rinsed with deionized

water to a pH of 5.3-5.5, and freeze dried before reweighing. The weight lost during this

process is assumed to be carbon dioxide from the decomposition of calcium carbonate,

allowing a determination of the carbonate content of the sediments.

The acid treated samples were freeze dried and weighed for stable isotope

measurements on a Costech EA coupled to a Finnigan MAT/Delta XP – isotope ratio

mass spectrometer (EA-irMS). Carbon and nitrogen isotope compositions are reported

on the VPDB and AIR scales respectively, using the standard per mil notation:

9


δ = (R Sample / R VPDB – 1) * 1000, where R Sample is the ratio of heavy isotope to light

isotope ( 13 C/ 12 C, 15 N/ 14 N) of the sample, and R VPDB is the ratio of heavy isotope to light

isotope of the standard. Precision of the measurements was based upon duplicate

analyses of 15 samples, +/- 1.05% for %OC, +/- 0.43 for C/N a , +/- 0.46‰ for δ 13 C OC and

+/- 0.30‰ for δ 15 N TN .

4.3 Chronology

The sediment chronology is based upon two accelerator mass spectrometry

radiocarbon (AMS 14 C) dates on terrestrial macrofossils (Table 1). The AMS 14 C dates

were converted to calendar years before present (ca. yr. BP) using the CALIB 5.0 dataset

(Reimer et al., 2004). The age-depth relationship was constructed by linear interpolation

between these two dates (Fig. 2).

10


5. Results

5.1 Age Model

The two AMS 14 C ages indicate that the study interval spans the period 11,273 to

13,757 cal. yr BP. The 14 C ages had an uncertainty of +/- 35 14 C yr BP, which translated

to an uncertainty interval in the calibrated ages of 11,399 to 11,611 cal yr BP and 12,892

to 12,965 cal yr BP. Sample ages were calculated by linear interpolation between the

radiocarbon ages (Fig. 2).

5.2 EA-irms Results

The samples were measured for weight percent of organic carbon (OC) on a

carbonate-free basis, weight percent of carbonate-free total nitrogen (TN), δ 13 C OC , and

δ 15 N TN . These weight percent OC and TN were used to calculate a weight percent and

atomic C/N ratio (Fig. 3). The results are used to divide the core into five zones; four

zones are from the results of weight percent organic carbon, while the fifth is based on an

excursion recorded in the isotope data, but not in the weight percent of organic carbon.

5.2.1 Weight Percent Carbon

The weight percent carbon from the sample core varied throughout the zones,

with the lowest values occurring during Zone I representing the Last Glacial Maximum

and rise into Zone V representing the early Holocene (Fig. 4).

Zone I begins with a weight percent OC value of 0.5% and fluctuates throughout

the zone before rising slightly at 45 cm to a value of 0.6% at the end of this zone. Zone II

continues the steady rise of wt. %OC to a value of 2.7% at 38 cm before dropping to

11


2.1% at 37 cm and then rising slightly to 2.6% at 36 cm at the end of this zone. This shift

can be more clearly seen in Fig 4. Zone III begins when the values rise rapidly to 7.6%

and reaches its peak of 10.6% at 34 cm before beginning to fall rapidly to 4.2% at 31 cm

which represents the end of this zone. This falling trend continues into Zone IV with a

value of 1.2% at 30 cm before rising in a stepwise fashion to 3.4% at 27 cm and then to

3.6% at 24 cm before falling slightly to 3.4% at 21 cm at the end of this zone. Zone V

representing the youngest portion of the study section, has an overall rising organic

carbon content, although there is a slight oscillation. The zone starts by rising rapidly to

12.7% at 17 cm before falling to 7.70% at 16 cm and then rising again to 14.7% the top

of the zone (12 cm).

5.2.2 Atomic Carbon/Nitrogen Ratio

The atomic carbon to nitrogen ratio (C/N a ) was calculated using the ratio of

analyzed weight percents of carbon and nitrogen multiplied by the ratio of the atomic

weights of nitrogen and carbon.

The C/N a values for Zone I beginning with a value of 13.1 at 54 cm before falling

to a value of 12.3 at 53 cm. The values then rise again to 13.3 at 50 cm before dropping

steadily to the end of the zone to a value of 11.3 at 45 cm. Zone II continues to see a

drop in C/N a values to 10.7 at 44 cm before rising rapidly to 14.7 at 42 cm. The ratio

then falls to 13.7 at 41 cm before rising again to 14.5 cm at 37 cm. The values then fall at

the end of the zone, 36 cm with a value of 13.9. Zone III begins by falling to 13.3 at 35

cm before rising steadily to 15.4 at the end of the zone which is a depth of 31 cm. Zone

IV continues the rise in C/N a values to a maximum of 16.0 at 26 cm before decreasing

12


steadily to 13.9 at 23 cm. The end of this zone is a rising to a value 14.8 at 21 cm. The

beginning of Zone V continues the increasing trend to a value of 15.2 at 19 cm, and then

drops off to 14.4 at 17 cm, but this value rises quickly to 15.0 at 16 cm before tapering

down to 14.5 at 12 cm which is the end of the zone.

5.2.3 Organic Carbon Isotope Results

Measured δ 13 C OC values varied between -18.4‰ and -32.6‰ in the core.

Beginning in Zone I, the δ 13 C rises from -22.0‰ to -20.8‰ at 53 cm before changing to a

more negative value of -25.8‰ at 48 cm. Values once again becomes more positive,

rising to -18.4‰ at 46 cm before abruptly shifting to -26.4‰ at 45 cm. Zone II continues

the negative trend to -30.6‰ at 41 cm before becoming more positive with a value of

-28.1‰ at 38 cm. The zone ends at 36 cm with a δ 13 C OC value of -31.7‰. Values in

Zone III continues to become more negative, with values reaching -32.3‰ at 32 cm and

then increasing to -30.9‰ at 31 cm, which is the end of the zone. Zone IV isotopic

values fluctuate, becoming more positive until -29.4‰ at 28 cm, and then becoming more

negative rapidly to -31.2‰ at 27 cm. The trend continues in this zone as the values once

again move towards the positive value of -29.5‰ at 25 cm before becoming negative

again to the end of the zone with a value of -32.4 at 21 cm. Zone V began by moving

towards values that are more negative until 19 cm when the δ 13 C OC value reached

-32.6‰. The values then moved positively towards -28.5‰ at 14 cm before quickly

becoming more negative (-30.6 at 12 cm) which marks the end of the zone.

13


5.2.4 Total Nitrogen Isotope Results

The δ 15 N TN results for the sample core oscillated throughout the different zones.

Zone I begins with a value of -0.15‰ at 54 cm before quickly jumping to a value of

9.76‰ at 53 cm. The values then become more negative to -0.59‰ at 51 cm before

rising once again to 7.37‰ at 47 cm. The oscillating trend continues as the values once

again becomes more negative, reaching -1.15‰ at 45 cm. Zone II begins by becoming

more positive, up to 0.39‰ and then dropping negatively to the value of -3.03‰ at 42

cm. There is then a positive movement to -1.11‰ at 38 cm before ending the zone with a

value of -2.07‰ at 36 cm. Zone III has values that fluctuate between -0.79‰ at 35 cm

and -0.84‰ at 31 cm which is the end of the zone. Zone IV begins with a value of

-1.67‰ at 30 cm that moves positively to 0.02‰ at 28 cm and then negatively back to

-1.21‰ at 25 cm. The values become more positive to -0.01‰ at 22 cm before moving

more negatively to -0.41‰ at 21 cm. The beginning of Zone V is marked by the

continue move to the negative value of -1.14‰ at 20 cm. The values then become

positive until 17 cm with a value of 0.75‰, they then fall back to 0.03‰ at 15 cm. The

end of this zone then sees a large positive rise to 1.78‰ at 13 cm before then falling to

1.59‰ at 12 cm.

14


6. Discussion

6.1 Weight Percent Organic Carbon

The weight percent of organic carbon is a useful tool to determine the input of

organic material preserved in lacustrine sediments. The presence of organic material can

be an indicator of organic productivity during past climate periods. The organic material

measured is a function of concentration based on the relative flux of organic carbon to the

flux of total material on a carbonate-free basis. The total flux of material includes the

flux of organic carbon, inorganic mineral material, and biogenic silica. While weight

percent is a general indicator of paleoproductivity, several other factors can affect the

amount of organic material. These factors include the lake morphology, watershed

topography, the sedimentation rate of non-organic carbon material, as well as the

abundance of terrestrial and aquatic flora.

Castor Lake is a closed basin lake situated on a topographic high allowing only the

input of organic material from nearby terrestrial or aquatic sources. The shallow depths

of the lake minimized oxidation and degradation of the organic material during sinking

because of the short exposures to the water column.

Zone I (~13757 – 13224 cal yr BP) is a period of low organic carbon. This zone

most likely represents a periglacial environment, with sparse watershed vegetation and

low aquatic productivity. This sparse terrestrial landscape would also provide greater

influx of mineral material into the lake that would keep the organic carbon values low.

Weight percent organic carbon, almost doubles from Zone I into Zone II (~13224 –

12692 cal yr BP). The rise in organic carbon can be attributed to increasing flux and

15


preservation of terrestrial organic matter. The rising organic carbon values in Zone II

could also represent a period of warming with increased lake and watershed organic

productivity. The climate change into a post-glacial world is one explanation for this

increase of organic carbon at the Zone II/III transition. This post-glacial period would be

warmer and wetter then the previous zone with increasing terrestrial productivity and

input. This increased terrestrial matter would also help stabilize the watershed soils and

prevent the input of inorganic mineral material.

During Zone III (~12692 – 12397 cal yr BP) organic carbon continues to rise to a

maximum of 10.6% at 34 cm (~12574 cal yr BP) before falling to 4.2% at the end of this

zone (31 cm, ~12397 cal yr BP). The trend observed in the beginning of this zone

represents the continued rise of organic carbon attributed to a greater input of organic

carbon, a decrease input from mineral material, or possibly both. The increasing trend at

the beginning of this Zone can be associated with the rising organic carbon values

observed in the post-glacial environment of Zone II. While the end of this zone marks a

significant drop in the organic carbon values, this lower organic carbon content was

possibly brought on by several factors including: a decrease in organic carbon influx,

either terrestrial or aquatic, a greater influx of mineral material, or both. One possible

explanation is a decrease in terrestrial plant productivity would allow greater mineral

influx from the loose watershed soils.

Zone IV (~12397 – 11746 cal yr BP), has low organic content, similar to values

observed at the end of Zone I and the beginning of Zone II. Cool and dry climate

conditions would decrease the terrestrial productivity and input of organic material

represented by the low levels of organic carbon observed in this zone. Alternatively, the

16


decrease in the organic carbon values can be attributed to a greater flux of mineral

material, which would be made possible by a decreased coverage of terrestrial flora. The

organic matter begins to rise at the end of the zone, indicating a rising influx of organic

carbon, or decrease in mineral material.

Zone V (~11747 – 11273 cal yr BP) continues the rising trend for the organic

matter to a value of 12.7%. The rise of organic content over this period from the end of

Zone IV through the beginning of Zone V represents the transition into the early

Holocene period. Although Zone V fluctuates between 12.7% organic material at 17 cm

(~11568 cal yr BP) to 7.7% at 16 cm (~11509 cal yr BP) before rising to 14.7% at the end

of the zone, 12 cm (~11273 cal yr BP), the overall trend is increasing from values

observed in Zone IV.

This fluctuation before the final rise represents the stabilization of the climate into

the early Holocene, although the changes in organic carbon are significant because they

fall outside the error values for these measurements. Either a drop in organic material

input into the lake, or a period of increased mineral input can explain the drop of percent

organic carbon from 17 cm to 16 cm. The rise to the end of the zone then marks either

increased productivity, increasing the organic material input, or a decrease in the mineral

input into the lake sediment record.

17


6.2 Atomic C/N

The atomic carbon to nitrogen ratio of organic matter varies with the deposition of

different organic material, whether it is lacustrine or terrestrial C 3 and C 4 plants.

Terrestrial plants increase the value of the C/N ratio, while aquatic algae decrease the

ratio. The typical C/N ratios from different inputs are summarized in figures 5 and 6.

This offers greater insight into the influx and preservation of organic material into the

lake sediments.

The C/N ratio for the Zone I have values that are low, beginning with 13.3 at 54

cm (~13756 cal yr BP) and falling to 11.3 at 45 cm (~13224 cal yr BP). These values

represent organic inputs that are mostly aquatic algae. A periglacial environment with a

sparse terrestrial landscape would account for these findings.

Zone II begins with a C/N ratio of 10.7, rises to 14.7 at 42 cm (~13046 cal yr BP),

fluctuates between 13.8 at 40 cm (~12928 cal yr BP) and 14.5 at 37 cm (~12751); ending

at 36 cm (~12692 cal yr BP) with a value of 14.0. Rising C/N values indicate a greater

influence due to the terrestrial plant material. This is supplemented by the weight percent

organic carbon data that rises during this zone.

C/N Values continue to increase in Zone III, reaching a maximum value of 15.4 at

the end of the zone. This rising values indicates either a drop in aquatic plant

productivity or a greater input and mixing of terrestrial land plant material. A warmer,

wetter climate from post glacial warming is a possible explanation that would provide the

increased terrestrial input necessary to cause the increased values.

The C/N values continue rising until 27 cm (~12160 cal yr BP), reaching a value

of 16.0, they then begin to fall to a value of 14.0 at 23 cm (~11923 cal yr BP). This

18


decline in the C/N ratio indicates that either there was a drop in terrestrial plant input or a

greater productivity from the aquatic flora. This trend coupled with the organic carbon

data suggest that the end of the zone is marked by a drop in productivity. An explanation

for the drop in C/N values is a cool, dry period characterized by less terrestrial input.

While these values fall to lower levels, they do not reach the levels represented by Zone I.

The C/N ratio rises to a value of 14.8 at the end of Zone IV; similar to the rise

seen at the end of Zone II and beginning of Zone III. C/N values rise from either an

increased input of terrestrial plant material, or decrease in aquatic plant matter. Values

continue to rise to 15.5 at the beginning of Zone V, which indicates further input from the

terrestrial sources, or decreased aquatic productivity. Values then oscillate to a value of

14.4 at before 17 cm (~11568 cal yr BP) before rising to 15.0 at 16 cm (~11509 cal yr

BP) and then falling again to 14.5 at the end of the zone. Zone V represents a period of

mixed organic input form both terrestrial and aquatic sources, that is expected for lakes

during this period of warming in a post-glacial climate.

The stabilization of the watershed soils with terrestrial material is illustrated with

a breakdown of both total organic carbon and total organic material with respect to

calcium carbonate and inorganic mineral material (Fig. 7). There is a trend of lessening

influence from inorganic mineral material from Zone I into Zone III, while the input of

organic and calcium carbonate increases. Also, during the Younger Dryas the organic

matter drops off tremendously while the value of inorganic mineral inputs remains

relatively the same as observed in Zone III.

19


6.3 Organic Carbon Stable Isotopes

The use of stable carbon isotopes contained within the zones offers a greater

understanding of the organic sources in and around Castor Lake. The isotopic signature

of the samples can help develop an understanding of the lake’s watershed and

depositional history because it enhances the ability to distinguish between aquatic and

terrestrial matter. The δ 13 C values of the sediment record are affected by the relative

amounts of terrestrial and aquatic organic matter. The δ 13 C of terrestrial organic matter

varies depending upon the vegetation type, with typical values for C 3 vegetation of -23‰

to -32‰, and C 4 of -8‰ to -17‰. The aquatic end-member can also vary depending

upon both the δ 13 C of the dissolved inorganic carbon and the rate of CO 2 utilization. The

δ 13 C value is affected by DIC because this is the main source of carbon for

photosynthesis, if the aquatic productivity increases such that it begins utilizing more

dissolved CO 2 in the water column, the δ 13 C values of the aquatic organic matter reflect

this and become more positive. Also, affecting the δ 13 C signal are exchanges with

sediment pore water, this is mainly controlled by inorganic mineral material (δ 13 C ≈ 0‰);

other influences also include precipitation (δ 13 C = -7‰ because it is in equilibrium with

atmospheric CO 2 ) (McKenzie, 1985; Meyers, 1997). Photosynthesis and respiration

affect the δ 13 C values based on source, with terrestrial values ranging from heavy to light

based on the metabolic pathway, while aquatic algae has δ 13 C values that are generally

lighter (McKenzie, 1985; Meyers, 1997). The use of δ 13 C is also a useful tool for

paleoclimate reconstruction because aquatic algae discrimination for 12 C increases when

20


the dissolved CO 2 concentration increases; lake water surface temperatures, and an

increase in the rate of organic carbon decomposition in the sediments increases the pCO 2

concentration (Meyers 1997).

The major influences into the system are most likely mineral material (δ 13 C ≈ 0

to-2.5‰) and terrestrial vegetation (mainly C 3 with δ 13 C ≈ -27‰) contained within the

catchment basin (Nelson 2004). Alteration in δ 13 C values from inflow is not a concern as

it is mostly from surface or subsurface flow contained within the small catchment.

Zone I begins with a δ 13 C org value of -22.0‰ at 54 cm (~13756 cal yr BP) before

becoming more positive at 53 cm (~13697 cal yr BP) with -20.8‰; this shift indicates a

possible greater input from a mixture of algae and C 3 terrestrial plants. However, the

C/N a value decreases in this interval indicating production of algal material. The more

positive δ 13 C org may represent greater productivity of the lacustrine algae and utilization

of a greater fraction of the dissolved CO 2 . The δ 13 C org values then become steadily more

negative to -25.8‰ at 48 cm (~13401 cal yr BP). This is a significant shift in the isotope

record, coupled with the C/N a ratio indicates a shift from input by algal material to more

terrestrial contributions.

The δ 13 C values then get significantly heavier at 46 cm (~13282 cal yr BP),

reaching -18.4‰, this change likely indicates continued input of algal material utilizing

more of the CO 2 within the water column. There is not a significant rise in the C/N a

during this period, supporting the assumption that the δ 13 C shift is due to greater algal

production. The carbon isotopic value then becomes more negative once again at 45 cm

to a value of -26.4‰. The δ 13 C and C/N a values characterize the organic input being

21


mainly from lacustrine algae with a decrease in aquatic productivity and less utilization

of dissolved CO 2 .

The transition from Zone I to Zone II represents a rapid shift in δ 13 C from -26.4 to

-30.2‰ and continues in the negative direction to -30.6‰ at 41 cm (~12987 cal yr BP).

This negative shift to lighter isotope values may be due to several factors, including

lower production by the aquatic algae, an increase in surface temperatures, or increased

input from terrestrial C 3 flora. There is a shift to a more positive value of -28.1‰ at 40

cm, before fluctuating back to -31.7‰ at 36 cm, the end of this zone. These values in

conjunction with C/N a changes, characterize this period as having greater input from

terrestrial C 3 plant derived organic material. An interesting relationship occurs when the

δ 13 C become more positive, the C/N a ratio drops. This shift may be accounted for by

greater productivity of by aquatic algae, accounting for a low C/N a value, with usage of a

greater fraction of the CO 2 in the water column driving the more positive δ 13 C values.

Zone III begins with a negative δ 13 C shift to -32.2‰ at 32 cm (~12455 cal yr BP),

when it then becomes more positive towards the end of the zone (31 cm, ~12396 cal yr

BP), with a value of -30.9‰. This positive shift along with an increase in the C/N a ratio

suggests that this period has a greater influence from terrestrial C 3 plants. Alternatively,

the positive shift of δ 13 C values may record the availability of dissolved CO 2 for the

aquatic algae (pCO 2 rises during periods of cold surface temperatures) or increased

aquatic productivity depleting the dissolved CO 2 (Meyers and Lallièr-Verges 1998).

Carbon isotopic signatures in Zone IV continue to move in the positive direction

reaching -29.4‰ at 28 cm (~12219 cal yr BP); this continued shift to the positive can be

a result of increased terrestrial input, which is reinforced by the C/N a values. The next

22


sample in the zone, 27 cm (~12159 cal yr BP) becomes more negative with a value of -

31.2‰, this value has a higher C/N a value, which could represent a greater input from

terrestrial C 3 plants, or less productivity by the aquatic algae. The δ 13 C values then

become more positive, reaching -29.4‰ at 25 cm (12041 cal yr BP), and corresponds

with a drop in the C/N a ratio that can represent a greater input from aquatic organisms.

The zone ends with a value of -32.4‰ at 21 cm (~11805 cal yr BP). During this

excursion of δ 13 C to more negative values, the C/N a ratio suggests that the input is from

lacustrine organic material. The more negative values can also be attributed to lower

surface temperatures which would increase the quantity of dissolved CO 2 (Meyers 1997).

The final zone begins with carbon isotopic values similar to the end of Zone IV,

but which become increasingly more positive at 16 cm (~11509 cal yr BP) where they

reach a value of -29.2‰. During this period, the C/N a values fall, which possibly

corresponds to aquatic algae production limited by low pCO 2 in the water column. The

δ 13 C then continues to become more positive to a value of -28.5‰ at 14 cm (~11391 cal

yr BP), before the zone ends with a value of -30.6‰ at 12 cm (~11273 cal yr BP).

During this zone the C/N a values rise, suggesting that this zone is represented by a greater

input of terrestrial C 3 vegetation.

6.5 Summary

The organic carbon, C/N a , and δ 13 C values all suggest Zone I begins with a sparse

terrestrial landscape and some aquatic algal material present in the lake. The end of the

zone is marked by a rise in algal production with greater use of the dissolved CO 2 in the

23


water. A periglacial environment with cooler temperatures is a likely scenario for this

zone.

Zone II continues with low organic carbon weight percents, although almost

double those observed in Zone I. This increase, coupled with the atomic C/N values and

δ 13 C values are all consistent with a greater input from terrestrial material, specifically

input from terrestrial C 3 plant material. The rising input from this terrestrial material may

also represent a greater stabilization of the watershed soils that limits the input from

inorganic mineral material. This zone can be characterized by post-glacial warming that

creates a more stable terrestrial landscape, causing greater input of terrestrial organic

matter into the lake. The middle of the zone has a noticeable δ 13 C excursion to more

positive values that represents an increase in aquatic productivity.

Zone III is notable because there is a rapid rise in weight percent of organic

carbon at the beginning of the zone, while the atomic C/N a values and δ 13 C values do not

fluctuate as greatly. These values present an interesting relationship, with data from

lower in the zone suggesting increased organic material input, including greater terrestrial

inputs possibly from the continued warming of Zone II. The organic carbon and C/N a

values help explain the increased productivity, both terrestrial and aquatic. Also, the lake

level during this period may have risen, allowing for stratification that would preserve

more organic material. The drop in organic carbon values at the end of this zone suggest

a cooling that may be responsible for decreasing the amount of terrestrial and aquatic

organic matter inputs in the lake. The C/N a ratio does not change during this interval

indicating that relative amounts of mixing between terrestrial and aquatic organic matter

inputs did not vary.

24


The organic carbon, atomic C/N a and δ 13 C values of Zone IV are consistent with

lower initial surface temperatures, and decreased terrestrial organic inputs. Also, like in

Zone III, lake level fluctuation may have played a role, with these values signifying a

drop in lake level, thus inhibiting preservation. This zone includes the Younger Dryas

cold reversal, and the organic carbon and δ 13 C data suggest the cooling observed with the

Younger Dryas. There is an interesting relationship between the δ 13 C and C/N a during

this zone; they remain relatively unchanged at the beginning of the zone and do no

experience a change until the middle of the zone where the C/N a drops, and the δ 13 C

becomes lower. This suggests that there is a delay from the start of the Younger Dryas

until there is a noticeable influence on the terrestrial vegetation. The end of the zone has

organic carbon, atomic C/N and δ 13 C values that correspond to continued low surface

temperatures, along with increased mineral input from a lack of terrestrial plant cover.

Zone V has a general rise in the organic carbon and atomic C/N ratio, these

values, along with positive δ 13 C values mark an influence from greater productivity of

both terrestrial and aquatic organic material, and represent the warming from the

Younger Dryas.

25


7. Conclusion

This study was conducted with the intent of observing a record of the Younger

Dryas abrupt climate reversal in the Northwest Pacific region of the United States. This

site was chosen because the dominant cause for the Younger Dryas, the thermohaline

circulation, is not the driving climatic force in this area. Castor Lake served as a good

source for preservation of organic carbon in this region, and yielded some interesting

information from the carbon and nitrogen isotope ratios.

The data shows that the changing climates affected this lake and watershed

system. The organic carbon, carbon isotopes and carbon to nitrogen ratio develop a story

of a changing ecosystem influenced by the climate. Minimal terrestrial coverage and a

growing aquatic algae input sustain the lake and surrounding area in the earliest section

of the core. As conditions warm into the Bølling-Allerød period, the terrestrial influences

become more dominant, and limit the input from inorganic mineral material; there is also

more input from aquatic algae as well. There is then a significant shift in the organic

carbon data and subsequent change to less terrestrial organic matter during the time

defined for the Younger Dryas. This illustrates the effect of this abrupt climate change

influencing regions not directly controlled by the thermohaline circulation.

This emphasizes previous research from the west coast region of the United States

and the resulting impact from the Younger Dryas. It also adds weight to the great

influence of this system on a scale that is more then just regional. Further research from

this site can yield better information on the severity of such things as the effect of abrupt

26


climate change on temperatures, moisture balance and organic material preservation on a

closed basin lake in this region.

27


Acknowledgements

First and foremost, I would like to thank Dr. Kate Freeman and Dr. Pratigya

Polissar, their knowledge and insight were invaluable towards my completion of this

project. They also helped to make this a great opportunity with advice that has allowed

me to grow exponentially as a scientist. Their support will not be soon forgotten and is

very much appreciated.

Thanks to Dr. David Bice, the Associate Head of the Undergraduate Program, for

his comments and advice during the writing process of this work.

Also, thanks must be extended to all the inhabitants of the 2 nd floor Dieke

Biogeochemistry lab your advice and support is much appreciated, as well as the Penn

State Geosciences Department for the opportunity and financial support to complete this

work.

In addition, thanks to Dr. Mark Abbott and Dan Nelson from the University of

Pittsburgh for providing the sample core and site information, and their funding provided

by the National Science Foundation’s Earth System History grant that allowed this

research.

28


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31


TABLE 1. RADIOCARBON DATES FROM CASTOR LAKE CORE SAMPLES

Sample

Depth

(cm)

AMS 14 C age

Median

calibrated age

(cal yr BP)

1-sigma

calibrated age

range

(cal yr BP)

Material

Lab accession

number

16 10025 +/- 35 11515 11399 - 11611 Seed UCI# 7491

40 11020 +/- 35 12937 12892 - 12965 Seed UCI# 7493

TABLE 2. CORE CHONOLOGY WITH SAMPLE INTERVALS AND

APPROXIMATE AGE

Zone

Sample Interval

(cm)

Age Interval

(cal yr BP)

I 45-54 13224 – 13757

II 36-45 12692 – 13224

III 31-36 12397 – 12692

IV 20-31 11746 – 12397

V 12-20 11273 – 11746

32


Figure 1.1 – Map outline of Washington showing Castor Lake as the black circle (Nelson

2004).

Figure 1.2 – Detailed satellite image of the Pacific Northwest, the study area is marked

by the thumbtack (http://earth.google.com/ Date Accessed: 5 April 2006)

33


Figure 1.3 – High resolution satellite image of the Castor Lake region.

(http://earth.google.com/ Date Accessed: 5 April 2006)

Figure 1.4 – Topographic map of the study site, Castor Lake resides on a topographic

high with an approximate elevation of 591 meters above sea level.

(http://www.topozone.com/ 5 April 2006).

34


Figure 2 – Plot of age versus depth for the Castor Lake sample core, the two radiocarbon

dates are represented by the open circles with error bars.

35


Figure 3 – Stable isotope data (δ 13 C OC , δ 15 N TN ), atomic C/N ratio values and weight

percent organic carbon data from the Castor Lake sample core, the Younger

Dryas is the region shaded. The two dashed lines indicate the location of the

two radiocarbon dates.

36


Figure 4 – Weight percent calcium carbonate and weight percent organic carbon data

from the Castor Lake sample core, the Younger Dryas is the shaded region.

The dashed lines indicate the location of the two radiocarbon dates.

37


Figure 5 – Stable carbon isotope data (δ 13 C OC ) plotted against atomic C/N ratio values

indicating the typical ranges where material is observed (Meyers 1998).

38


Figure 6 – Stable carbon isotope data (δ 13 C OC ) plotted against atomic C/N ratio values

from the Castor Lake sample core. The Younger Dryas occurred at the end of

Zone III and the entirety of Zone IV.

39


Figure 7 – Breakdown of total organic matter (TOM), calcium carbonate (CaCO 3 ), and

inorganic mineral/biogenic silica input for each zone.

40


APPENDIX A

Mean Element and Isotope Data

Sample (Depth cm) %C %CaCO 3 %TN δ 13 C C/Na δ 15 N

12 14.75 - 1.18 -30.65 14.51 1.59

13 13.29 - 1.08 -30.01 14.40 1.78

14 8.36 - 0.66 -28.54 14.78 1.31

15 9.91 60.71 0.78 -29.70 14.82 0.03

16 7.70 53.19 0.60 -29.17 15.05 0.35

17 12.67 52.48 1.03 -30.69 14.39 0.75

18 9.44 96.69 0.74 -31.77 14.89 1.45

19 9.63 57.36 0.74 -32.65 15.24 0.20

20 6.25 34.38 0.47 -32.31 15.50 -1.14

21 3.37 19.22 0.27 -32.40 14.85 -0.41

22 3.38 46.78 0.26 -32.04 14.99 -0.07

23 3.59 70.89 0.30 -30.26 13.91 -0.12

24 3.58 67.67 0.28 -30.17 14.76 -0.60

25 2.73 40.95 0.22 -29.47 14.41 -1.21

26 3.19 13.88 0.24 -30.75 15.60 -0.57

27 3.36 14.28 0.25 -31.19 15.96 -0.76

28 1.32 11.68 0.10 -29.40 15.14 0.02

30 1.22 13.10 0.09 -29.61 15.10 -1.67

31 4.23 13.48 0.32 -30.87 15.37 -0.84

32 8.31 18.54 0.66 -32.21 14.70 -0.38

33 10.64 24.86 0.84 -31.55 14.68 -0.68

35 7.63 19.13 0.67 -31.64 13.29 -0.79

36 2.59 19.91 0.22 -31.67 13.95 -2.07

37 2.11 18.87 0.17 -29.17 14.46 -2.04

38 2.72 75.42 0.23 -28.14 14.26 -1.11

39 1.72 - 0.11 -28.07 17.99 -2.88

40 1.67 12.45 0.14 -28.87 13.76 -1.29

41 1.38 - 0.12 -30.65 13.74 -1.03

42 1.17 11.00 0.09 -30.57 14.69 -3.03

43 1.41 10.97 0.13 -30.59 13.09 0.28

44 0.96 6.91 0.10 -30.24 10.74 0.39

45 0.62 3.51 0.06 -26.43 11.31 -1.15

46 0.30 5.84 0.03 -18.36 12.02 -0.48

47 0.49 4.94 0.05 -22.80 11.39 7.37

48 0.82 5.44 0.08 -25.78 12.24 1.43

49 0.79 5.20 0.07 -23.50 12.54 1.19

50 0.77 4.13 0.07 -23.14 13.34 0.69

51 0.42 3.62 0.04 -23.19 12.08 -0.59

52 0.51 3.28 0.05 -21.65 12.10 5.64

53 0.28 3.99 0.03 -20.84 12.29 9.76

54 0.54 3.35 0.05 -21.98 13.13 -0.15

41

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