A pervasive link between Antarctic ice core and subarctic Pacific ...

S.L. Jaccard et al. / Quaternary Science Reviews 29 (2010) 206–212 207
In contrast, dense waters are exposed at the surface of highlatitude
oceans, such as the subarctic Pacific (Gargett, 1991; Talley,
1995) and the Southern Ocean (Sarmiento et al., 2004), and are
returned to the subsurface before the available nutrients are fully
utilized by phytoplankton for carbon fixation (Sarmiento and
Toggweiler, 1984; Sigman and Boyle, 2000). The exposure of
nutrient-rich deep water at the surface fuels phytoplankton
growth, but simultaneously it allows CO 2 previously sequestered by
the biological pump to be released to the atmosphere. The uptake of
nutrients and CO 2 during formation of phytoplankton biomass
through photosynthesis and the eventual export of organic matter
subsequently lowers the pCO 2 of surface waters, causing the surface
layer to reabsorb a portion of the CO 2 that was initially released
from the upwelled water. However, any unused nutrients that are
physically pumped back into the ocean interior as ‘preformed’
nutrients represent a shortfall in the potential efficiency of the
biological pump (Sigman and Haug, 2003; Ito and Follows, 2005)
Thus, it is the ratio between export production and the ventilation
rate of CO 2 -rich subsurface waters (not the absolute rate of either
process alone) that controls the net exchange of CO 2 between the
atmosphere and the high-latitude surface ocean (Sarmiento and
Toggweiler, 1984). As a result, either increased export flux and/or
reduced communication between the subsurface ocean and the
atmosphere would contribute to sequester a larger proportion of
CO 2 in the ocean during ice ages, thereby contributing to reduce
atmospheric CO 2 concentrations (Stephens and Keeling, 2000;
Sarmiento and Toggweiler, 1984; Siegenthaler and Wenk, 1984;
Sigman and Boyle, 2000).
1.2. The subarctic NW Pacific – past and present
The subarctic Pacific is one of the three major high nutrient low
chlorophyll (HNLC) regions of the world ocean, characterized by
upwelling of nutrient-rich deep water into the euphotic zone due to
Ekman divergence (Gargett, 1991). Low ambient iron concentrations
limit complete utilization of available dissolved nutrients and
growth of large celled phytoplankton (Tsuda et al., 2003). Macronutrient
supply to the photic zone is moderated by the strength of
the permanent salinity-driven density gradient (halocline) that
dominates the region today. The presence of this low-salinity
surface layer prevents thorough convection; hence no deep water is
formed today in the open subarctic Pacific (Emile-Geay et al., 2003;
Warren, 1983). In addition, the relatively effective isolation of the
subtropical and subarctic gyres minimizes dilution of the high
nutrient water by nutrient-depleted waters from lower latitudes. In
spite of rather limited mixing between mid-depths and the surface
ocean, the modern subarctic Pacific still constitutes a source of
oceanic CO 2 to the atmosphere (Takahashi et al., 2002) despite the
anthropogenically-elevated atmospheric partial pressure of CO 2 .
Primary productivity is dominated by siliceous algae (diatoms) in
early summer and to a lesser degree by calcareous phytoplankton
early autumn when silicic acid concentrations have dropped.
Sediment trap observations have shown that total mass flux and
biogenic opal flux are highly correlated (Honda et al., 2002), suggesting
that opal is the main export vector for organic carbon
(Otosaka and Noriki, 2005). At station KNOT (44 N, 150 E), the
transfer efficiency of organic carbon (i.e. the ratio between organic
carbon flux to primary productivity) was estimated to be approx.
3% at 3000 m water depth, a value that is substantially higher than
observed elsewhere in the world ocean (Honda et al., 2002).
There seems to be a general consensus, that export production
in the NW subarctic Pacific was significantly lower during peak ice
ages when compared to interglacial periods (Brunelle et al., 2007;
Galbraith et al., 2008; Gebhardt et al., 2008; Kienast et al., 2004;
Jaccard et al., 2005, 2009; Narita et al., 2002; Shigemitsu et al.,
2007), suggesting that nutrient supply from below was reduced
during cold periods. A corollary to these observations is that the
physical transfer of deeply sequestered CO 2 to the surface ocean
through upwelling and turbulent mixing would have been reduced
during ice ages, thereby contributing to lower global atmospheric
CO 2 concentrations.
In this manuscript, we present new high-resolution measurements
from a sedimentary archive retrieved from the abyssal NW
subarctic Pacific Ocean. The time resolution achieved here rivals the
measurement density typical for Antarctic ice-core records. We
build upon previous observations (Galbraith et al., 2008; Jaccard
et al., 2005, 2009) by extending the Ba/Al, Ca/Al and biogenic opal
records back to 800 kyr to allow comparison with EPICA Dome C
(EDC) records. Our multi-proxy approach reveals that export
production and thus the nutrient supply to the surface is linked to
Antarctic dD and CO 2 records on millennial timescales, suggesting
a strong physical link between temperature in high southern latitudes
and the nutrient supply to the subarctic Pacific photic zone.
Our reconstructions further document the contribution the
subarctic Pacific made to modulate the partitioning of CO 2 between
the abyssal ocean and the atmosphere on glacial-interglacial
timescales.
2. Material, methods & proxies
2.1. Core material
ODP Site 882 is located in the subarctic NW Pacific (Detroit
Seamount, 50 21 0 N, 167 35 0 E; water depth 3244 m). The core
contains undisturbed diatomaceous ooze sequences with episodic
carbonate-rich intervals and scattered ash layers (Rea et al., 1993).
The astronomically calibrated stratigraphy for ODP Site 882 was
generated based on fine tuning of GRAPE (Gamma Ray Attenuation
Porosity Evaluator) density oscillations in the orbital precession
band to the summer insolation at 65 N(Tiedemann and Haug,
1995). In addition, benthic foraminiferal oxygen isotope records
from ODP Sites 882 and 883 (cross-correlated with magnetic
susceptibility) in intervals with CaCO 3 were used to corroborate the
tuned stratigraphy. The age model was linearly interpolated
between the selected tie-points. To derive the detailed age model
we then fine-tuned the ODP Site 882 Ba/Al record to the EDC dD
record (Jouzel et al., 2007) by visually matching common inflection
points and interpolating linearly between them (Galbraith et al.,
2008; Jaccard et al., 2005). This approach assumes a strict in-phase
relationship between EDC dD and ODP 882 Ba/Al records, which is
unlikely to be true, and therefore introduces some degree of
uncertainty. Bioturbation in these oxic sediments is in the order of
a few centimeters, smoothing the records slightly. Sedimentation
rates typically range between 5 and 10 cm*kyr 1 .
2.2. Analytical methods
Relative sedimentary elemental concentrations (Al, Ca, Ba) were
measured with an Aavatech profiling X-ray Fluorescence (XRF) core
scanner at Bremen University at a 1–3 cm (i.e. submillenial) resolution.
The calculations of normative Ba/Al and Ca/Al are based on
the assumption that the Al, Ba and Ca content of the terrigenous
material remained constant in space and time. Comparison
between discrete ICP-MS and coulometric determination of sedimentary
CaCO 3 and relative Ba/Al and Ca/Al counts, respectively,
has shown a highly significant statistical correlation (Jaccard et al.,
2009; Suppl. Fig.). Biogenic opal was quantified by alkaline
extraction of silica (Mortlock and Froelich, 1989) for the time
interval back to 150 kyr (Jaccard et al., 2009) and with help of the
density separation technique on carbonate- and organic carbon-