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Late Holocene storm-trajectory changes inferred from the oxygen ...

Late Holocene storm-trajectory changes inferred from the oxygen ...

J PaleolimnolTable 1

J PaleolimnolTable 1 Characteristics and surface-water isotope values for surveyed lakes located within 210 km of Mica LakeSite name/code aCollectiondateLatitude(°N)Longitude(°W)Elevation(m)Climatezone bLaketypeHydrologicsetting cd 18 O(%VSMOW)dD(%VSMOW)Robe Aug-2007 61.085 146.172 9 Maritime Non-glacial Open -15.9 -127Shrode Jun-2006 60.652 148.330 24 Maritime Non-glacial Open -14.0 -99Turner Jun-2006 60.910 146.627 33 Maritime Non-glacial Open -14.4 -107Portage Aug-2007 60.846 148.987 54 Maritime Glacial Open -14.9 -112Miners Jun-2006 61.079 147.436 55 Maritime Glacial Open -16.4 -123Milliard Jun-2006 60.906 146.578 80 Maritime Non-glacial Open -15.3 -113Silver Jun-2006 60.930 146.462 95 Maritime Glacial Open -15.6 -116‘‘Mica’’ Jun-2006 60.692 148.149 100 Maritime Non-glacial Open -12.9 -98‘‘Mica pond’’ Aug-2007 60.694 148.148 103 Maritime Non-glacial Closed -8.6 -63Bear (Seward Pen) Aug-2007 60.191 149.358 111 Maritime Glacial Open -14.4 -111‘‘Cascades’’ Jun-2006 60.930 147.869 180 Maritime Glacial Open -14.8 -108Trail Aug-2007 60.489 149.368 180 Maritime Glacial Open -17.2 -134Tern Aug-2007 60.535 149.545 221 Maritime Non-glacial Open -16.5 -128Lower Paradise Jun-2006 60.344 149.074 290 Maritime Non-glacial Open -16.0 -120Jerome Aug-2007 60.546 149.570 300 Maritime Non-glacial Closed -15.6 -128Nellie Juan Jun-2006 60.248 149.045 350 Maritime Glacial Open -15.9 -117‘‘Allison’’ Jul-2006 61.040 146.349 413 Maritime Glacial Open -16.7 -124Summit Aug-2007 60.636 149.504 435 Maritime Non-glacial Open -17.5 -134Willow Aug-2007 61.787 145.184 435 Interior Non-glacial Closed -13.3 -129Long Aug-2007 61.805 148.234 468 Interior Non-glacial Closed -13.9 -128‘‘GRS-1’’ Aug-2007 62.106 145.471 470 Interior Non-glacial Closed -16.8 -151‘‘GRS-2’’ Aug-2007 61.522 145.226 570 Transitional Non-glacial Closed -15.0 -138Blueberry Aug-2007 61.121 145.697 610 Transitional Non-glacial Closed -17.2 -136Pippin Aug-2007 61.714 145.161 621 Interior Non-glacial Closed -13.0 -128Plumbob Aug-2007 62.107 145.945 624 Interior Non-glacial Closed -14.8 -134Tolsona Aug-2007 62.109 146.037 630 Interior Non-glacial Open -13.5 -131‘‘Ptarmigan’’ Aug-2007 61.148 145.723 705 Transitional Non-glacial Closed -18.5 -139‘‘Lee’’ Jul-2006 62.085 146.395 760 Interior Non-glacial Closed -16.1 -143‘‘Goat’’ Jul-006 60.260 149.905 840 Maritime Non-glacial Open -15.9 -123Greyling Jul-2006 61.392 145.736 1000 Transitional Glacial Open -20.6 -154‘‘Eureka’’ Aug-2007 61.939 147.168 1010 Interior Non-glacial Closed -15.7 -138‘‘Hallet’’ Jul-2006 61.494 146.238 1160 Transitional Glacial Open -21.5 -160a ‘‘ ’’ indicates informal nameb Papineau 2001c Topographic setting closed = no surface outflowfrom the littoral zone when logistical constraintslimited boat access. The bottles were kept frozen orcool whenever possible.Hydrogen and oxygen isotope ratios of watersamples were measured with a Thermo-FinniganDeltaplus XL gas isotope-ratio mass spectrometerinterfaced to a Thermo-Electron Gasbench IIheadspace equilibration device at the ColoradoPlateau Stable Isotope Laboratory at NorthernArizona University. Analytical precision on internalworking standards was ±1% for dD and ±0.1% ford 18 O and is reported as per mil deviations from theinternational V-SMOW (Standard Mean OceanWater) standard.123

J PaleolimnolSediment core recovery, lithology,and geochronologyIn June 2006, percussion cores (7.6 cm diameter upto 3.1 m long) and companion surface gravity cores(6.5 cm diameter up to 0.3 m) were recovered fromthree sites in Mica Lake. The percussion (MC-2) andsurface core (MC-2C) from site 2 (hereafter referredto collectively as ‘MC-2’) at 58 m depth (Fig. 4), isthe longest recovered sediment sequence and is thefocus of this study. Coring at this site was presumablyhalted by the bedrock as evidenced by freshly brokengranite fragments lodged in the bottom of thepercussion core. Cores were split, photographed,and stored at 4°C. Sediment characteristics and tephrahorizons were described and assigned a Munsell soilcolor. Samples for isotope, biogenic silica (BSi), andorganic-matter (OM) analyses, were collected atcloser intervals up-core to achieve higher resolutionduring the last two centuries (0–5 cm, contiguoussampling) and the last 2000 years (10–100 cm, 5 cmspacing), with coarse-sampling for the remainder ofthe core (100–300 cm, 10 cm spacing). BSi wasmeasured using a wet-alkaline extraction technique(Mortlock and Froelich 1989) and organic matter wasmeasured by loss-on-ignition.For radiocarbon analyses, 0.5-cm-thick sampleswere collected at 20 cm spacing, or where vegetationmacrofossils were visible, sieved and dried under alaminar-flow hood. AMS 14 C analyses were performedat the Keck Carbon Cycle AMS Facility,University of California, Irvine or the LawrenceLivermore National Laboratory, Berkeley, California.210 Pb and 137 Cs were measured on two gravity cores,but did not provide interpretable geochronologicalinformation (Schiff 2007).Diatom floraA preliminary diatom stratigraphy was generatedfrom the Mica Lake sequence to assess the extent thatd 18 O diatom correlates with diatom floristic composition.Diatoms were examined every 20 cm (i.e. 15samples), thus capturing each phase of Holocene lakedevelopment, albeit at coarse resolution. Diatomsamples (200 mg dry sediment) were processed withsuccessive digestions in 10% and 30% H 2 O 2 .Digested slurries were diluted and strewn ontocoverslips, dried at room temperature, and mountedto slides with NaphraxÒ medium. Taxonomic resolutionwas limited to genus level (Round et al. 1990)because this investigation aimed to assess only majorshifts in the diatom community.Relationships between diatom assemblage compositionand sediment geochemical variables wereevaluated objectively using principal componentsanalysis (PCA), an indirect linear ordination method(ter Braak and Prentice 1988). PCA was applied to acorrelation matrix including the relative frequenciesof the dominant diatom genera observed (Achnanthes,Aulacoseira, Encyonema, Eunotia, Fragilariasensu lato, Frustulia, Navicula, and Pinnularia) aswell as corresponding OM, BSi, and d 18 O diatomvalues for the 15 core depths under consideration.Pre-screening of the data indicated very short firstaxis gradients (i.e. 0.49 standard deviation units),attesting to the suitability of linear ordinationapproaches. The ordination was conducted usingCANOCO software running under Windows (terBraak and Smilauer 1998).Diatom isolation and analysesDiatoms from 46, 0.5-cm-thick samples were isolatedand analyzed for their oxygen isotope composition(d 18 O diatom ). Pure diatoms must be isolated becauseother lake sediment components (e.g. tephra, mineralgrains, organic matter) contain oxygen that isliberated during fluorination of the sample for massspectrometer analyses (Leng and Barker 2006). Athree-stage protocol, involving chemical digestion,wet sieving, and heavy-liquid separation, was adaptedfrom Morley et al. (2004). Diatom extracts werescreened for purity using a high-powered microscope,and a scanning electron microscope (SEM) was usedto further confirm the purity of selected intervals(Fig. 5). The 10- to 50-lm size fraction was thepurest.The silica of diatom frustules includes an outerhydrous layer and a denser inner layer. Oxygen in theinner layer is isotopically homogenous, but the outerlayer freely exchanges with the ambient water afterformation (Juillet 1980). Therefore, diatom silicamust be dehydrated prior to mass spectrometryanalyses to obtain an isotope value representative ofthe lake water at the time of frustule formation.Numerous methods have been used to remove theouter hydrous layer, including vacuum dehydration,123

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