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

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

J Paleolimnolrelative to

J Paleolimnolrelative to ambient water (Brandriss et al. 1998),implying that additional isotopic discrimination processesexist in natural environments that are notcaptured in culturing experiments.Controls on d 18 O diatom in southern AlaskaNon-climatic controlsDiagenetic effects have been reported in studies thatcompare the oxygen isotope ratios in diatomscollected in traps versus those extracted from sediment(e.g. Moschen et al. 2006). These studies founda slow-acting maturation of diatoms that led toenrichment of values after deposition. The authorsattribute the enrichment to silica dissolution anddehydroxylation and suggest that d 18 O diatom recordswith long-term trends of enrichment with age shouldconsider diagenetic effects as a leading explanation.Backscattered electron microscope images show noobvious dissolution of modern (Fig. 5a; 0 cm) ormid-Holocene (Fig. 5b; 60 cm) diatom frustules fromMC-2. This observation, combined with the lack of along-term trend in the MC-2 record suggests thatdiagenetic effects are absent or undetectable at MicaLake.Diatom habitat and taxonomic (vital) effects mightinfluence d 18 O diatom values. Because diatoms live inboth benthic and planktonic settings, they experiencedifferent temperatures through most of the year.Benthic and planktonic diatoms formed at the sametime could have different isotope values, particularlyif the lake water d 18 O value or temperature weredifferent between habitats. At Mica Lake, however,surface and bottom water during June 2006 andAugust 2007 had nearly identical d 18 O values,suggesting the lake is well mixed during the latespring and summer.In Mica Lake, the three zones recognized in thediatom assemblages are entirely coherent with thed 18 O diatom record, suggesting that the same climaticfactors that regulated lake water and diatom d 18 O alsoinfluenced the composition of diatom assemblage. Ofthe diatom genera encountered, Aulacoseira andEunotia are the most closely associated with theloading vector of d 18 O diatom on the first 2 PCA axes,in positive and negative associations, respectively(Fig. 9a). These results are consistent with theexpansion of planktonic and tychoplanktonic generasuch as Aulacoseira under relatively warm conditionswith prolonged open-water seasons (Smol et al.2005), as well as with increased representations ofacidophilic diatoms (e.g. Eunotia spp.) during coldintervals (Wolfe 2002).There is, however, no evidence that diatomassemblage composition exerts a first-order controlover d 18 O diatom values. Diatom genera other thanAulacoseria and Eunotia appear unrelated to thevector of d 18 O diatom in PCA, with several generabeing orthogonal to this gradient (Achnanthes,Frustulia, and Pinnularia). The stratigraphy ofsummed benthic diatoms, excluding Achnanthes andEunotia (Fig. 9b), confirms this point: these taxaincrease initially as d 18 O diatom values decline after4.0 ka, but continue to expand during the subsequentrecovery of d 18 O diatom in the last millennium.We therefore surmise that the stratigraphic trendsof Eunotia and Aulacoseira are predictable ecologicalresponses to the climate shifts suggested by the MicaLake d 18 O diatom record, rather than driving theisotopic signal through species-specific differencesin 18 O/ 16 O fractionation. These results are compatiblewith experimental studies that revealed scant evidencefor metabolic differences in oxygen isotopefractionation among diatom genera (Brandriss et al.1998) although Swann et al. (2008) found evidence ofa vital effect in marine diatoms.Climatic controlsThe inability of water temperature—diatom fractionationand non-climatic factors to fully explain thelarge variability of Holocene d 18 O diatom values suggeststhat the d 18 O value of Mica Lake water hasvaried over the last 9600 cal BP. The two primarycontrols on lake water d 18 O are the P/E balance andthe d 18 O of the inflow, both of which vary withclimate.P/E balanceThe P/E balance strongly influences dW of lakes(Gat 1981). dW data from the lakes surveyed in thisstudy exemplify these influences: interior lakes,receiving low precipitation are hydrologically closed(Table 1), and evaporation at these sites causesenrichment in D and 18 O, which shift values off the123

J PaleolimnolGMWL (Fig. 6d) (Gat 1981). In contrast, lakes in thePrince William Sound receive an average of about3 m of precipitation a year, and dW is influenced onlyslightly by evaporation. At Mica Lake, surface andbottom waters collected in June 2006 and August2007 suggest a small progressive isotopic enrichmentof surface water from summer evaporation: d 18 Ovalues of bottom water are nearly identical betweensummers (-13.2 and -13.4%), whereas the surfacewater from August 2007 is ?0.5% enriched relativeto June 2006. The enrichment is small whencompared with the late Holocene ?4.6% range ofd 18 O diatom at Mica Lake, and might be irrelevant ifdiatom blooms always occur during the late spring,prior to progressive evaporation through the summermonths. Furthermore, surface water inflow into MicaLake collected during the summer is indistinguishablefrom Mica Lake surface water.Using a range of precipitation rates from 3 to7 m year -1 , which encompasses the observed rate atWhittier during the last 25 year (average = 5.5 myear -1 ), and assuming that evapotranspiration is lessthan 0.5 m year -1 , which is typical for south Alaska(Newman and Branton 1972), the lake water residencetime is \5 year at Mica Lake. Lakes with such lowresidence time are relatively unaffected by evaporation(Leng and Marshall 2004).Taken together, the high precipitation input, shortwater residence time, nearly homogenous dW at MicaLake, and overlap with the local meteoric water line(Fig. 6), suggest that the modern dW is not affectedstrongly by evaporation, and instead is controlledlargely by dP. This conclusion is supported by thelong-term d 18 O diatom record from Mica Lake. The dWof lakes is typically elevated and variable whenevaporation exerts a strong control, whereas dW isgenerally low and relatively constant during wetperiods (Leng and Marshall 2004). At first glance,therefore, the increased late Holocene variability ind 18 O diatom at Mica Lake (2.6 ka to modern; Fig. 10a)could reflect increased evaporation. On the other hand,the shift to lower d 18 O diatom values following 2.6 kaand the coincidence with the increased variability isinconsistent with the hypothesis of increased evaporation.We cannot entirely rule out evaporative effectsduring the early and middle Holocene, but the mostrecent d 18 O diatom value is the highest (?29.8%), so anychanges in P/E are likely masked by stronger controls.Isotope analyses of diatom samples containing othersediment components (e.g. tephra, mineral grains)(Brewer et al. 2008) would yield lower values, but ourSEM screening did not reveal contamination.Seasonality of precipitationSeasonal differences in dP could affect dW if theseasonality of precipitation has varied. The relativelysparse IAEA-GNIP data available for the NorthPacific region (Fig. 1 inset) provide some insightinto the seasonal range of dP in south Alaska. Themaritime climate at Mica Lake is most similar to theAdak IAEA-GNIP station, located 2000 km to thewest (Fig. 1 inset). IAEA-GNIP data collectedbetween 1962 and 1973 record mean summer (JJA)temperature at Adak that is ?9°C higher than winter(DJF) temperatures, whereas mean d 18 O is ?0.4%higher in summer compared to winter. Applying the18 O—air temperature fractionation from the IAEA-GNIP global dataset (?0.65% °C -1 ; Rozanski et al.1993) to the ?9°C seasonal temperature rangeimplies a range of ?5.9% between summer andwinter, which is an order of magnitude greater thanwhat is recorded. The amount of precipitation, on theother hand, is more strongly correlated to d 18 O valuesin the Adak dataset: monthly d 18 O is inversely relatedto the amount of precipitation (r = - 0.50, p \ 0.1).If the Adak data are representative of conditions atMica Lake, higher d 18 O diatom might representdecreased precipitation, but is likely insensitive tochanges in seasonality of precipitation because theseasonal range of d 18 O is low. The general similaritybetween climate at Adak and the Prince WilliamSound is not necessarily a good indication that dPfrom the two areas should correlate, however. Tofurther investigate the ‘‘amount effect’’ in the NorthPacific, a common phenomenon at low latitudes,year-long collections are needed.Air temperatureThe global relationship between dP and temperature(i.e. ‘Dansgaard effect’) ranges between *?0.2 and?0.9% °C -1 (Rozanski et al. 1993). Applying theseslopes to the range in d 18 O diatom at Mica Lake(*?4.6%) suggests a shift of ?5 to ?23°C duringthe late Holocene. The upper estimate is not plausible,whereas the lower estimate is similar to the DJFtemperature range observed between the ten strongest123

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