Alexander P. Wolfe

A Coherent Signature of Anthropogenic Nitrogen Deposition to Remote Watersheds of the Northern Hemisphere

Deposition of reactive nitrogen from human activities occurred in the preindustrial era. Humans have more than doubled the amount of reactive nitrogen (Nr) added to the biosphere, yet most of what is known about its accumulation and ecological effects is derived from studies of heavily populated regions. Nitrogen (N) stable isotope ratios (15N:14N) in dated sediments from 25 remote Northern Hemisphere lakes show a coherent signal of an isotopically distinct source of N to ecosystems beginning in 1895 ± 10 years (±1 standard deviation). Initial shifts in N isotope composition recorded in lake sediments coincide with anthropogenic CO2 emissions but accelerate with widespread industrial Nr production during the past half century. Although current atmospheric Nr deposition rates in remote regions are relatively low, anthropogenic N has probably influenced watershed N budgets across the Northern Hemisphere for over a century.

Anthropogenic nitrogen deposition induces rapid ecological changes in alpine lakes of the Colorado Front Range (USA)

Recent sediments from two alpine lakes (> 3300 m asl) in the Colorado Front Range (USA) register marked and near-synchronous changes that are believed to represent ecological responses to enhanced atmospheric deposition of fixed nitrogen from anthropogenic sources. Directional shifts in sediment proxies include greater representations of mesotrophic diatoms and increasingly depleted δ15N values. These trends are particularly pronounced since ~ 1950, and appear to chronicle lake responses to excess N derived from agricultural and industrial sources to the east. The rate and magnitude of recent ecological changes far exceed the context of natural variability, as inferred from comparative analyses of a long core capturingthe entire 14,000-year postglacial history of one of the lakes. Nitrogen deposition to these seemingly pristine natural areas has resulted in subtle but detectable limnological changes that likely represent the beginning of a stronger response to nitrogen enrichment.

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

Mid-Miocene cooling and the extinction of tundra in continental Antarctica

A major obstacle in understanding the evolution of Cenozoic climate has been the lack of well dated terrestrial evidence from high-latitude, glaciated regions. Here, we report the discovery of exceptionally well preserved fossils of lacustrine and terrestrial organisms from the McMurdo Dry Valleys sector of the Transantarctic Mountains for which we have established a precise radiometric chronology. The fossils, which include diatoms, palynomorphs, mosses, ostracodes, and insects, represent the last vestige of a tundra community that inhabited the mountains before stepped cooling that first brought a full polar climate to Antarctica. Paleoecological analyses, 40Ar/39Ar analyses of associated ash fall, and climate inferences from glaciological modeling together suggest that mean summer temperatures in the region cooled by at least 8°C between 14.07 ± 0.05 Ma and 13.85 ± 0.03 Ma. These results provide novel constraints for the timing and amplitude of middle-Miocene cooling in Antarctica and reveal the ecological legacy of this global climate transition.

Over three millennia of mercury pollution in the Peruvian Andes

We present unambiguous records of preindustrial atmospheric mercury (Hg) pollution, derived from lake-sediment cores collected near Huancavelica, Peru, the largest Hg deposit in the New World. Intensive Hg mining first began ca. 1400 BC, predating the emergence of complex Andean societies, and signifying that the region served as a locus for early Hg extraction. The earliest mining targeted cinnabar (HgS) for the production of vermillion. Pre-Colonial Hg burdens peak ca. 500 BC and ca. 1450 AD, corresponding to the heights of the Chavín and Inca states, respectively. During the Inca, Colonial, and industrial intervals, Hg pollution became regional, as evidenced by a third lake record ≈225 km distant from Huancavelica. Measurements of sediment-Hg speciation reveal that cinnabar dust was initially the dominant Hg species deposited, and significant increases in deposition were limited to the local environment. After conquest by the Inca (ca. 1450 AD), smelting was adopted at the mine and Hg pollution became more widely circulated, with the deposition of matrix-bound phases of Hg predominating over cinnabar dust. Our results demonstrate the existence of a major Hg mining industry at Huancavelica spanning the past 3,500 years, and place recent Hg enrichment in the Andes in a broader historical context.

Recent Changes in the Diatom Community Structure of Lakes in the Beartooth Mountain Range, U.S.A.

Abstract In alpine lakes from several regions of the world, sedimentary diatom profiles indicate that rapid shifts in diatom community structure have occurred over the past century. A number of these recent shifts have been attributed to anthropogenic disturbances such as enhanced atmospheric nitrogen (N) deposition or climate change. When these disturbances are coupled, the response of alpine lakes becomes more complex and varies from region to region. The Beartooth Mountain Range, situated on the border between Montana and Wyoming, is part of the central Rocky Mountains; it is considered an area of relatively low N deposition but has experienced an increase in bulk precipitation rates, primarily in the form of snowfall, over the past century. We have examined a 400-yr sediment record from Beartooth Lake and have observed a rapid change in the diatom community structure over the past decade. A typical alpine lake diatom flora, consisting mainly of small Fragilaria sensu lato species, dominated this lake until approximately 1995, at which time Fragilaria crotonensis and Cyclotella bodanica var. lemanica rapidly increased to approximately 30% each of the total assemblage. The diatom assemblages from the tops and bottoms of short cores from three additional lakes in the area also reveal taxonomic shifts. These shifts appear indicative of both increased N loading to these systems as well as changes in thermal stratification patterns.

Quantitative paleotemperature estimates from d 18 O of chironomid head capsules preserved in arctic lake sediments

A paleoenvironmental perspective of temperature change is paramount to understanding the significance of recent warming in the Arctic. Late Quaternary sediments from many arctic lakes provide environmental archives with decadal resolution, but reconstructions are hampered by the relative insensitivity of many traditional proxies to temperature. Here, we show that the δ18O of head capsules of chironomid larvae are equilibrated with the δ18O of lakewaters in which they live. In suitable lakes, lakewater δ18O is controlled by the δ18O of local precipitation, which is strongly correlated to mean annual air temperature (MAT). From this correlation, chironomid δ18O can be used to examine past changes in MAT. We illustrate the potential of this novel approach to paleothermometry with examples from two arctic lakes that reveal strong regional paleoclimatic gradients in the early Holocene.

Global change revealed by palaeolimnological records from remote lakes: a review

Over recent decades, palaeolimnological records from remote sites have provided convincing evidence for the onset and development of several facets of global environmental change. Remote lakes, defined here as those occurring in high latitude or high altitude regions, have the advantage of not being overprinted by local anthropogenic processes. As such, many of these sites record broad-scale environmental changes, frequently driven by regime shifts in the Earth system. Here, we review a selection of studies from North America and Europe and discuss their broader implications. The history of investigation has evolved synchronously with the scope and awareness of environmental problems. An initial focus on acid deposition switched to metal and other types of pollutants, then climate change and eventually to atmospheric deposition-fertilising effects. However, none of these topics is independent of the other, and all of them affect ecosystem function and biodiversity in profound ways. Currently, remote lake palaeolimnology is developing unique datasets for each region investigated that benchmark current trends with respect to past, purely natural variability in lake systems. Fostering conceptual and methodological bridges with other environmental disciplines will upturn contribution of remote lake palaeolimnology in solving existing and emerging questions in global change science and planetary stewardship.

Stratified interglacial lacustrine sediments from Baffin Island, Arctic Canada: chronology and paleoenvironmental implications

Thirteen of 18 piston cores recovered from ‘Robinson Lake’ in the mid-Arctic vegetation zone of Baffin Island, Arctic Canada, penetrated stratified lacustrine sediment beneath a thin over-consolidated diamict (till) and postglacial lacustrine sediment. The sub-till lacustrine units are up to 120 cm thick, of which the upper several decimeters frequently contain thick, layered mats of aquatic moss; pollen and diatoms are common throughout both lacustrine units. A series of 23 AMS 14C dates defines the chronology of the postglacial sequence, which records a succession from a pioneer grass- and Oxyria-dominated tundra between 10.4 and 8 ka BP, to a sedge-dominated tundra after 8 ka BP. Limiting 14C dates place the sub-till lacustrine sediments more than 40 ka BP; 10 luminescence dates centered on 85 ka indicate they were deposited late in oxygen isotope stage (OIS) 5. The dominance of shrub and tree pollen, especially shrub birch and alder, in sub-till lacustrine samples, indicates low-arctic tundra farther north than at any time during the Holocene. Pollen concentrations are comparable to or higher than in the Holocene units. Cooling late in the interglacial is indicated by declining birch and alder pollen percentages in the upper part of the section. Diatom floras in both the sub-till and postglacial lacustrine sequences indicate similar development of lake-water chemistry, but input of silica and weathering products was greater in the older lake cycle, suggesting more vigorous catchment processes. Macrofossils in the sub-till units are characteristic of lakes ice-free in summer. Based on pollen assemblages indicating local and regional vegetation diagnostic of summer temperatures higher than the Holocene, we interpret the sub-till lacustrine units to be of interglacial character. By analogy with Holocene plant succession in central and eastern Canada, all of Keewatin and Labrador/Ungava must have been ice free throughout this interval, suggesting essentially complete deglaciation of the Laurentide Ice Sheet at the time.

Climate modulates the acidity of Arctic lakes on millennial time scales

High-resolution diatom stratigraphies from two lakes on eastern Baffin Island (Nunavut, Canadian Arctic) are used to reconstruct lake-water pH over the past 5 k.y. Despite contrasting geomorphic histories and markedly different diatom floras, the inferred pH of both lakes declined in concert with Neoglacial cooling. This result confirms that climate exerts a first-order influence on the acidity of these poorly buffered oligotrophic lakes, through links between lake ice cover, primary productivity, and dissolved inorganic carbon dynamics. Diatom-based pH inferences from dilute lakes can therefore assist paleoclimatic interpretations from lake sediments.