Brent B. Wolfe

Holocene paleohydrology and glacial history of the central Andes using multiproxy lake sediment studies

Here we document at century to millennial scale the regional changes of precipitation^evaporation from the late Pleistocene to present with multiproxy methods on a north^south transect of lake sites across the eastern cordillera of the central Andes. The transect of study sites covers the area from V14‡S to 20‡S and includes core studies from seven lakes and modern calibration water samples from twenty-three watersheds analyzed to constrain the down-core interpretations of stable isotopes and diatoms. We selected lakes in different hydrologic settings spanning a range of sensitivity to changes in the moisture balance. These include: (1) lakes directly receiving glacial meltwater, (2) overflowing lakes in glaciated watersheds, (3) overflowing lakes in watersheds without active glaciers, and (4) lakes that become closed basins during the dry season. The results of our current work on multiple lakes in the Bolivian Andes show that while the overall pattern of Holocene environmental change is consistent within the region, conditions were not always stable over centennial to over millennial timescales and considerable decadal- to centuryscale climate variability is evident [Abbott et al., Quat. Res. 47 (1997) 70^80, Quat. Res. 47 (1997) 169^180, Quat. Sci. Rev. 19 (2000) 1801^1820; Polissar, Master’s thesis, University of Massachusetts (1999)]. Comparison of the paleoclimate record from one well-studied site, Lago Taypi Chaka Kkota (LTCK), with others within the region illustrates a consistent overall pattern of aridity from the late glacial through the middle Holocene. Previous work noted a difference between the timing of water-level rise in Lake Titicaca V5.0^3.5 ka B.P. [Abbott et al., Quat. Res. 47 (1997) 169^180; Cross et al., Holocene 10 (2000) 21^32; Rowe et al., Clim. Change 52 (2002) 175^199] and the onset of wetter conditions at 2.3 ka B.P. in LTCK, a lake that drains into the southern end of Lake Titicaca [Abbott et al., Quat. Res. 47 (1997) 70^80]. Sedimentary and oxygen isotope evidence from Paco Cocha (13‡54PS) located in the northern reaches of the expansive 57 000 km 2 Titicaca watershed, which spans V14‡S to 17‡S, indicates that

Carbon and Oxygen Isotope Analysis of Lake Sediment Cellulose: Methods and Applications

Carbon and oxygen isotope analysis of lake sediment cellulose is a recently developed paleolimnological approach that is gaining increasing usage, especially in carbonate-free sedimentary systems. As with carbonate-based paleolimnological investigations (see Ito, this volume), lake sediment cellulose can provide a record of lake paleohydrology. As a result, studies incorporating this technique typically aim to address the following research questions:

Changes in carbon and nitrogen cycling during tree-line retreat recorded in the isotopic content of lacustrine organic matter, western Taimyr Peninsula, Russia

Bulk organic and cellulose stable carbon isotope and bulk organic nitrogen isotope profiles from the sediments of a small tundra lake on the western Taimyr Peninsula, Russia, show changes that are correlated with climate cooling and treeline retreat at c. 4000 14C years BP. Increased soil organic matter decomposition, combined with a moist climate, probably provided a 13C-depleted source of CO2(aq) to lake phytoplankton thriving under favourable conditions during the forest interval. Increased concentration of CO2(aq) and re-utilization of respired CO2(aq) from organic matter settling through the water column may have also contributed to the relatively low δ13C values in the lower part of the lake sediment record. Bulk organic and cellulose carbon are more enriched in 13C after the boreal forest retreated probably because CO2 from the atmosphere became the dominant source of carbon to the lake as soil organic decomposition rates declined, the lake became more oligotrophic, and the climate became colder and drier. Reduced concentration of CO2(aq) may have also led to 13C-enrichment. Interpretation of the bulk organic δ15N record is somewhat more speculative but changes in drainage basin terrestrial vegetation, soil decomposition and hydrology also appear to have strongly influenced the lake water nitrogen cycling. Although considerably more effort is required to assess modern carbon and nitrogen isotope systematics, these promising results suggest that lake sediment organic matter δ13C, δ15N and cellulose δ13C combine to form useful tracers of past nutrient cycling in boreal tree-line watersheds.

Holocene hydrological reconstructions from stable isotopes and paleolimnology, Cordillera Real, Bolivia

Multiproxy analyses of sediment cores from Lago Taypi Chaka Kkota (LTCK) Cordillera Real, Bolivia, provide a record of drier conditions following late Pleistocene deglaciation culminating in pronounced aridity between 6.2 and 2.3 ka B.P. Today LTCK is a glacial-fed lake that is relatively insensitive to changes in P}E because it is largely bu!ered from dry season draw-down through the year-round supply of glacial meltwater. This was not the case during the middle to late Holocene when glaciers were absent from the watershed. Lake-water d18O values inferred from d18O analysis of sediment cellulose range from !12.9 to !5.3& and average !8.7& between 6.2 and 2.3 ka B.P. Modern lake-water d18O from LTCK averages !14.8& which is compatible with the d18O -8 value of !14.3& for the surface sediment cellulose. Analyses of d18O from modern surface waters in 23 lakes that span the range from glacial-fed to closed basin vary from !16.6 to !2.5&. This approximates the magnitude of the down-core shift in d18O -8 values in LTCK during the middle to late Holocene from !12.9 to !5.3&. Strong paleohydrologic change during the middle Holocene is also evident in diatom assemblages that consist of shallow-water, non-glacial periphytic taxa and bulk organic d13C and d15N that show increases likely resulting from degradation of lacustrine organic matter periodically exposed to subaerial conditions. Neoglaciation began after 2.3 ka B.P. as indicated by changes in the composition of the sediments, lower d18O values, and a return to diatom assemblages characteristic of the glacial sediments that formed during the Late Pleistocene. Collectively, these data indicate that the past 2.3 ka B.P. have been the wettest interval during the Holocene. Millennial-scale shifts in the paleohydrologic record of LTCK during the early to middle Holocene conform to other regional paleoclimatic time-series, including Lake Titicaca and Nevado Sajama, and may be driven by insolation and resultant changes in atmospheric circulation and moisture supply. In contrast, an apparent 1200-year lag in the onset of wetter conditions at LTCK (2.3 ka B.P.) compared to Lake Titicaca (3.5 ka B.P.) provides evidence for variable sub-regional hydrologic response to climate change during the middle to late Holocene. ( 2000 Elsevier Science Ltd. All rights reserved.

Reconstruction of paleohydrology and paleohumidity from oxygen isotope records in the Bolivian Andes

Cellulose-inferred lake water N 18 O( N 18 Olw) records from Lago Potosi (LP), a seasonally closed lake in a watershed that is not currently glaciated, and Lago Taypi Chaka Kkota (LTCK) [previously reported in Abbott et al., 2000. Quat. Sci. Rev. 19, 1801^1820], an overflowing lake in a glaciated watershed, provide the basis for late Pleistocene and Holocene paleoclimatic reconstruction in the Bolivian Andes. Deconvolution of the histories of changing evaporative isotopic enrichment from source water N 18 O in the lake sediment records is constrained by comparison to the Sajama ice core oxygen isotope profile, whereas local hydrological influence is distinguished from the regional moisture balance history by the response of the different catchments to climate change. Overall, variations in the LP N 18 Olw record appear to be dominantly controlled by evaporative 18 O-enrichment, reflecting shifts in local effective moisture. This record is used to generate a preliminary quantitative reconstruction of summer relative humidity spanning the past 11 500 cal yr on the basis of an isotope-mass balance model. Results indicate that the late Pleistocene was moist with summer relative humidity values estimated at 10^20% greater than present. Increasing aridity developed in the early Holocene with maximum prolonged dryness spanning 7500^6000 cal yr BP at LP, an interval characterized by summer relative humidity values that may have been 20% lower than present. Highly variable but dominantly arid conditions persist in the mid- to late Holocene, with average summer relative humidity values estimated at 15% below present, which then increase to about 10^20% greater than present by 2000 cal yr BP. Slightly more arid conditions characterize the last millennium with summer relative humidity values ranging from 5^10% lower than present. Similar long-term variations are evident in the LTCK N 18 Olw profile, except during the early Holocene when lake water evaporative 18 O-enrichment in response to low relative humidity appears to have been offset by enhanced inflow from 18 O-depleted snowmelt or groundwater from the large catchment. Although some temporal offset is evident, significant correspondence occurs between the isotope-inferred paleohumidity reconstruction and other paleohydrological proxies from the region. These

Reassessment of Lake Victoria–Upper Nile River paleohydrology from oxygen isotope records of lake-sediment cellulose

Reconstruction of lake-water δ 1 8 O from analysis of cellulose δ 1 8 O in two sediment cores from Lake Victoria, East Africa, shows a large shift to lower values during the terminal Pleistocene. This shift records the transition from closed- to open-basin conditions following desiccation at the Last Glacial Maximum. Although oxygen isotope analysis of cellulose from one core had placed this overflow at 8 ka (7200 1 4 C yr B.P.), reevaluation of the age model for this core, in addition to new stratigraphic and chronological evidence from a second core, suggests that basin overflow was established much earlier, ca. 13 ka. Our refined view of the timing of Lake Victoria overflow inferred from the oxygen isotope records is consistent with other paleolimnological studies, indicating that lake-sediment cellulose is an effective and sensitive isotopic archive of major hydrologic events in this region.

Hamilton Harbour, Ontario: 8300 years of limnological and environmental change inferred from microfossil and isotopic analyses

Limnological changes in Hamilton Harbour, Lake Ontario, over the Holocene were investigated by using proxy evidence from diatoms and other siliceous microfossils in a radiometrically dated sediment core (HH26comp), together with environmental data derived from sediment pollen and oxygen and carbon isotope analyses. The evidence demonstrates that the site of Hamilton Harbour has changed over the past 8300 y from a shallow, separate waterbody, to a deep embayment of Lake Ontario. The earliest evidence, from 8300 BP to 7000 BP, is of a mesotrophic pond of moderate alkalinity, warmer than present, and probably with an extensive marginal wetland. An initial transitory connection with the rising water level of Lake Ontario was established at c. 7000 BP, possibly via a deep outlet channel. This connection is 2000 y earlier then previously estimated. Permanent confluence with Lake Ontario was established at c. 6200 BP, causing a decline in inferred trophic level and water temperatures. Microfossils reach a minimum at 4400 BP coincident with the Nipissing Flood. Decreased mixing of Lake Ontario water from about 4000 BP following the Nipissing Flood highstand is evidenced in isotopic and diatom data. Three isolated shifts in the diatom spectrum at c. 4900 BP, 4500 BP, and 3500 BP may be associated with extreme turbidity or storm deposit events. Between 3200 BP and 280 BP, Hamilton Harbour was evidently a moderately alkaline embayment of Lake Ontario, oligotrophic to mesotrophic, and relatively cooler than present. The final 280 y sedimentary record reveals the magnitude of anthropogenically induced changes to the harbour, including eutrophication and organic pollution.

Hydrologic control on the oxygen-isotope relation between sediment cellulose and lake water, western Taimyr Peninsula, Russia: Implications for the use of surface-sediment calibrations in paleolimnology

Systematic variability occurs between the oxygen isotopic composition of lake water sampled in mid-summer 1993 and cellulose extracted from surficial sediments of a suite of lakes spanning the forest-tundra transition near Norilsk, Russia. Some tundra and all forest-tundra lakes show greater deviation from expected cellulose-water isotopic separation than forest lakes, apparently because of greater sensitivity to 18O-depleted snowmelt contributions. Cellulose derived from aquatic plants naturally integrates fluctuations in lake water δ18O, providing a signal that is inherently more representative of average thaw season lake water δ18O than the measure of instantaneous δ18O obtained from an individual sample of lake water. Thus, indiscriminate use of empirical cellulose-water relations derived from ‘calibration’ samples could lead to erroneous assessment of paleohydrology from the oxygen-isotope stratigraphy of sediment cores from arctic lakes. However, deviation from the expected cellulose-water fractionation is a source of lake-specific hydrologic information useful for qualifying paleoenvironmental interpretations and possibly constraining non-isotopic methods that rely on surface-sediment calibrations.

A 6000-year record of interaction between Hamilton Harbour and Lake Ontario: quantitative assessment of recent hydrologic disturbance using 13C in lake sediment cellulose

Abstract Hamilton Harbour, a heavily urbanized and polluted embayment, has been selected for environmental remedy by an International Joint Commission. Ecosystem restoration efforts, however, require an understanding of harbour water balance and in particular the influence of recently enhanced exchange with the more dilute waters of Lake Ontario via the Burlington Canal. Here we provide a 6000-year quantitative reconstruction of hydrologic communication between Hamilton Harbour and Lake Ontario, based on the carbon isotope composition in the cellulose of the lake sediment, as a tracer of dissolved inorganic carbon. Results indicate that excavation of the canal has led to mixing levels 30–100% greater than the natural hydrologic state, conditions comparable to the Nipissing Flood when Upper Great Lakes drainage was diverted through Lakes Erie and Ontario roughly 5000 years ago. The effects of elevated exchange are clearly displayed by abrupt attenuation of anthropogenically driven eutrophication in the upp...

Biotic responses to multiple aquatic and terrestrial gradients in shallow subarctic lakes (Old Crow Flats, Yukon, Canada).

Biotic communities in shallow northern lakes are frequently used to assess environmental change; however, complex interactions among multiple factors remain understudied. Here, we present analyses of a comprehensive data set that evaluates the influence input waters, catchment characteristics, limnology, and sediment properties on diatom and chironomid assemblages in surface sediments of ~49 shallow mainly thermokarst lakes in Old Crow Flats, Yukon. Multivariate analyses and ANOSIM tests identified that composition of diatom (119 taxa) and chironomid (68 taxa) assemblages differs significantly (p < 0.05) between lakes with snowmelt- versus rainfall-dominated input water. Redundancy analyses revealed strong correlation of limnological, sediment, and catchment variables with input waters. Variation partitioning analyses showed that unique effects of limnological variables account for the largest proportion of variation in diatom and chironomid assemblages (17.2% and 12.6%, respectively). Important independe...