Paleohydrological Proxies Research Articles

Multiple paleohydrological proxies for western Mongolia based on lacustrine ostracodes

Multiple paleohydrological proxies for western Mongolia based on lacustrine ostracodes

Molecular Paleohydrology: Interpreting the Hydrogen-Isotopic Composition of Lipid Biomarkers from Photosynthesizing Organisms

Hydrogen-isotopic abundances of lipid biomarkers are emerging as important proxies in the study of ancient environments and ecosystems. A decade ago, pioneering studies made use of new analytical methods and demonstrated that the hydrogen-isotopic composition of individual lipids from aquatic and terrestrial organisms can be related to the composition of their growth (i.e., environmental) water. Subsequently, compound-specific deuterium/hydrogen (D/H) ratios of sedimentary biomarkers have been increasingly used as paleohydrological proxies over a range of geological timescales. Isotopic fractionation observed between hydrogen in environmental water and hydrogen in lipids, however, is sensitive to biochemical, physiological, and environmental influences on the composition of hydrogen available for biosynthesis in cells. Here we review the factors and processes that are known to influence the hydrogen-isotopic compositions of lipids—especially n-alkanes—from photosynthesizing organisms, and we provide a framework for interpreting their D/H ratios from ancient sediments and identify future research opportunities.

Rapid, time-transgressive, and variable responses to early Holocene midcontinental drying in North America

The end Pleistocene to early Holocene drying of the North American midcontinent is an important case study for understanding regional drought, its drivers, and its impacts. This paper synthesizes pollen, δ13C, eolian, and paleohydrological proxies to document the spatial and temporal patterns in the onset and rate of drying. The timing of onset ranged from 14 to 6 ka with most sites drying between 10 and 8 ka (calendar years ago). The dominant spatial pattern is time transgressive, with interior Great Plains sites beginning to dry before peripheral sites. This time-transgressive trend can be explained as a region-wide drying that caused ecotones to shift eastward, or could indicate a progressive shift in atmospheric drying from west to east. One-third of sites responded rapidly to drying; a cluster of rapid responses occurred ca. 8 ka. The combination of onsets between 14 and 6 ka and rapid responses at 8 ka strongly suggests that (1) the combination of high summer insolation, Laurentide Ice Sheet retreat, and Lake Agassiz drainage drove midcontinental drying, and (2) drying accelerated with Laurentide Ice Sheet collapse at 8.4 ka. Local factors strongly mediate the timing and rate of site responses to regional drying. Local responses to future drying in the Great Plains also should be highly variable, which challenges impact assessments of future climate changes.

Position of the Polar Front along the western Iberian margin during key cold episodes of the last 45 ka

This paper documents the migration of the Polar Front (PF) over the Iberian margin during some of the cold climatic extremes of the last 45 ka. It is based on a compilation of robust and coherent paleohydrological proxies obtained from eleven cores distributed between 36 and 42°N. Planktonic δ18O (Globigerina bulloides), ice‐rafted detritus concentrations, and the relative abundance of the polar foraminifera Neogloboquadrina pachyderma sinistral were used to track the PF position. These three data sets, compared from core to core, show a consistent evolution of the sea surface paleohydrology along the Iberian margin over the last 45 ka. We focused on five time slices representative of cold periods under distinct paleoenvironmental forcings: the 8.2 ka event and the Younger Dryas (two recent cold events occurring within high values of summer insolation), Heinrich events 1 and 4 (reflecting major episodes of massive iceberg discharges into the North Atlantic), and the Last Glacial Maximum (typifying the highest ice volume accumulated in the Northern Hemisphere). For each event, we generated schematic maps mirroring past sea surface hydrological conditions. The maps revealed that the Polar Front presence along the Iberian margin was restricted to Heinrich events. The sea surface conditions during the Last Glacial Maximum were close to those at present day, except for the northern sites which briefly experienced subarctic conditions.

Holocene palaeohydrology and climate variability in northeastern Spain: The sedimentary record of Lake Estanya (Pre-Pyrenean range)

This multi-proxy study of sediment cores from karstic Lake Estanya (Pre-Pyrenean Range, NE Spain) provides the first complete, continuous record of the hydrological evolution in the northeastern Iberian Peninsula over the last 9500 yr. Six sedimentary facies and four main sedimentary units have been defined after integration of sedimentological, mineralogical and geochemical analyses. The use of total sulphur content and sedimentary facies as paleohydrological proxies allows reconstruction of relative changes in lake level. The Estanya record shows a large increase in water availability after 9.2 ka, fluctuating lake levels and salinity during the period 9.2–4.2 ka; and generally higher lake levels after 1.7 ka. Periods of increased run-off and sediment delivery and less saline conditions occurred at 8.5–8.2, 6.7–5.9, and 4.9–4.2 ka. Dominant lower lake levels and concentrated waters during the period 4.2–0.8 ka were punctuated by a higher lake level, higher clastic input episode ca. 1.7–1 ka. Fluctuating, but higher lake levels occurred during the last 800 years. The main hydrological phases in Lake Estanya are coherent with Western Mediterranean and North Atlantic Holocene reconstructions, but they also show similarities with northern African records.

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

Cellulose-inferred lake water δ18O (δ18Olw) 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 δ18O 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 δ18Olw record appear to be dominantly controlled by evaporative 18O-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 δ18Olw profile, except during the early Holocene when lake water evaporative 18O-enrichment in response to low relative humidity appears to have been offset by enhanced inflow from 18O-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 results support the contention that millennial-scale variations in tropical moisture balance may be linked to changes in summer insolation.

Holocene lake sediment records of Arctic hydrology

Although paleoclimatic research in the Arctic has most often focused on variations in temperature, the Arctic has also experienced changes in hydrologic balance. Changes in Arctic precipitation and evaporation rates affects soils, permafrost, lakes, wetlands, rivers, ice and vegetation. Changes in Arctic soils, permafrost, runoff, and vegetation can influence global climate by changing atmospheric methane and carbon dioxide concentrations, thermohaline circulation, and high latitude albedo. Documenting past variations in Arctic hydrological conditions is important for understanding Arctic climate and the potential response and role of the Arctic in regards to future climate change. Methods for reconstructing past changes in Arctic hydrology from the stratigraphic, isotopic, geochemical and fossil records of lake sediments are being developed, refined and applied in a number of regions. These records suggest that hydrological variations in the Arctic have been regionally asynchronous, reflecting the impacts of different forcing factors including orbitally controlled insolation changes, changes in geography related to coastal emergence, ocean currents, sea ice extent, and atmospheric circulation. Despite considerable progress, much work remains to be done on the development of paleohydrological proxies and their application to the Arctic.