Variations in the oxygen isotope composition of ancient

Climate of the Past, 2009

Variability in water isotopes has been captured in numerous archives and used to infer past climate changes. Here we examine water isotope variability over the course of the Holocene using the water-isotope enabled, coupled atmosphere-ocean general circulation model, GISS ModelE-R. Eight Holocene time slices, ∼1000 years apart are simulated and driven by estimated changes in orbital configuration, greenhouse gases, and ice sheet extent. We find that simulated water isotope archives match well with those seen in ice cores, ocean sediment cores, and speleothems. The climate changes associated with the water isotope changes, however, are more complex than simple modern spatial slope interpretations might suggest. In particular, water isotope variability in Asian speleothems is linked to alterations in landward water vapor transport, not local precipitation, and ice sheet changes over North America lead to the masking of temperature signals in Summit, Greenland. Salinity-seawater isotope variability is complicated by inter-ocean basin exchanges of water vapor. Water isotopes do reflect variability in the hydrology, but are better interpreted in terms of regional hydrological cycle changes rather than as indicators of local climate. 1 Introduction Water isotopes are important tracers of the hydrologic cycle. In the atmosphere, the oxygen isotopic composition of precipitation, δ 18 O prec (δ in per mil units ≡−Rsample/Rstd−1×1000-refers to the ratio of the sample concentration to a known standard) is a product of the initial composition of δ 18 O in the water vapor of an air parcel and the amount of rain out, evaporation of rainfall,

Quaternary Science Reviews, 2011

We examine here variations in the organic carbon and nitrogen contents and isotopic compositions of Lake Superior sediments over the last 10,600 years, using bulk organic matter from four cores distributed across the Lake Superior Basin. Very low Organic Carbon (OC) and Total Nitrogen (TN) contents but high C org Mass Accumulation Rates (MARs) characterize these sediments until glacial meltwater supply to the Basin ended at w9000e8700 cal BP. The C/N ratios for organic matter from the glacial sediments span the range known both for organic matter adsorbed on soil clays and lacustrine algae grown under conditions of nitrogen deficiency. Organic matter in the glacial sediments has more-or-less uniform carbon-isotope compositions (wÀ27&), perhaps associated with a steady flux of soil clays. Coexisting ostracode valves vary by in carbon isotopic composition by as much as 5&, particularly between 10,600 and 10,400 cal BP, reflecting changing DIC sources at this time.

Paleoceanography, 2013

1] Records of temporal variability in the oxygen isotopic composition of biogenic carbonates (ı 18 O c ) from ocean sediment cores inform our understanding of past continental ice volume and ocean temperatures. Interpretation of ı 18 O c variability typically neglects changes due to factors other than ice volume and temperature. Here we investigate whether glacial-interglacial changes in sea ice, which fractionates seawater during its formation, could shift the isotopic value of seawater-in the deep ocean (affecting benthic foraminiferal ı 18 O c and thereby potentially impacting oxygen isotope based sea level reconstructions) or in surface waters (affecting planktic foraminiferal ı 18 O c ). We simulate glacial and interglacial states with the isotope-enabled University of Victoria Earth System Climate Model and perform a global analysis. Distinct patterns of sea ice production emerge for the glacial versus interglacial states. We find no substantive shift in ı 18 O w in model deep or bottom waters due to the simulated interglacial-glacial sea ice variability. Small isotopic shifts due to sea ice variability are concentrated in the model's surface waters of the Northern Hemisphere, specifically in the Labrador Sea and northeastern North Atlantic. (2013), Impact of sea ice variability on the oxygen isotope content of seawater under glacial and interglacial conditions, Paleoceanography, 28, 388-400,

Quaternary Science Reviews, 2010

Multi-component isotopic analyses on a sediment core from Lake Saarikko, Finland, give insight into Holocene climate changes in northern Europe. Oxygen isotopic records from aquatic cellulose and authigenic carbonate reflect consistent, similar trends in hydrological balance, and are employed jointly to infer relative changes in lake water temperature. Other geochemical and isotopic tracers shed light on fluctuations in lake level and nutrient cycling, facilitating the interpretation of the oxygen isotopic archives. In the early Holocene (w10 000e9000 cal yr BP), the lake level was low and dry conditions prevailed despite low temperatures. Higher lake levels and depleted oxygen isotopic values w9000e7500 cal yr BP were probably caused by several factors: higher effective humidity, an isotopically depleted precipitation source, a cold event around 8300 cal yr BP, and increased spring snowmelt. In contrast, higher summer temperatures, lower effective humidity, lower lake levels and enriched oxygen isotopic values were experienced during the mid-Holocene ∼7500e3300 cal yr BP. The occurrence of the warmest inferred lake water and the most enriched cellulose oxygen-isotope values ∼6000e5000 cal yr BP is consistent with a summer temperature maximum inferred from other proxy sources from the region. The late Holocene was characterized by higher effective humidity and higher lake levels. Increase in spring snowmelt and colder temperatures, most likely related to the Little Ice Age, induced a drastic change in the lake hydrology and probably caused the formation of its present outlet ∼800 cal yr BP. Changes in the lake hydrological balance and climate variables inferred from oxygen isotopic records can be related to atmospheric circulation, most clearly to changes in the dominance and strength of westerly air-flow as well as summertime anticyclonic circulation.

Geology, 1981

Oxygen-isotope analyses of the benthic foraminiferal species Uvigerina peregrina show that glacially induced changes in the isotopic composition of sea water reached a maximum of 1.5%o in late Pleistocene time in Tanner Basin, southern California. Isotopic data derived from the planktonic species Globigerina quinqueloba, corrected for glacial and regional water-mass effects, indicates that an increase in mean surface-water temperatures of about 5 °C took place from near the end of the Wisconsin glaciation to the Holocene thermal maximum at about 7,500 yr B.P. The isotopic minimum recorded by U. peregrina at about 40,000 yr B.P. is unusually low compared to that found in open-ocean cores.

Earth and Planetary Science Letters, 2010

This study presents osmium (Os) isotope and elemental data for cleaned planktic foraminifera, authigenic Fe-Mn oxyhydroxides and pelagic carbonate host sediments from ODP site 758 in the southernmost reaches of the Bay of Bengal. The Os in the bulk sediments appears to be dominantly hydrogeneous (sourced by carbonate and Fe-Mn oxyhydroxide), but variations in this particular core are controlled by the presence of volcanic ash. Fe-Mn oxyhydroxide leachates (of the bulk sediments) from Holocene samples also yield an Os isotope composition close to that of seawater, but the record diverges from that of foraminifera at a depth corresponding to the oxic/post-oxic boundary, suggesting diagenetic mobilization of Os at depths below this. Holocene planktic foraminifera, cleaned using oxidative-reductive techniques, also give Os isotope compositions indistinguishable from modern seawater, but the record obtained for the past 150 kyr shows strong covaraitions of 187Os/ 188Os with both the local and global oxygen isotope record, with less radiogenic Os isotope compositions during glacial intervals. These results indicate that foraminifera provide a robust record of seawater Os isotope compositions, and comparison of the data obtained here with records from the other major oceans demonstrate global changes in 187Os/ 188Os over this time interval, while the covariation with oxygen isotopes suggest a process controlling the Os isotope composition that is in phase with global climate cycles. Global excursions to relatively unradiogenic 187Os/ 188Os during glacial intervals are consistent with decreased input of radiogenic continental material, reflecting cooler temperatures and reduced continental runoff. Modelling indicates that the shift to unradiogenic values during glacial intervals could be caused by an ˜ 30% decrease in the global river flux, with an ˜ 5% change in river composition. If the residence time of Os in the oceans is ˜ 5 ka then the post-glacial recovery to present-day seawater values is consistent with a corresponding increase in the river flux of around 30%. However, if the residence time of Os is closer to 40 ka, as is suggested by the global river flux, then this demands either significant changes in both the riverine Os flux and composition of around 40% and 30%, respectively, that closely follow the oxygen isotope record, or else a short-lived post-glacial pulse of weathering some 75% greater than the steady-state flux. In either case, these results clearly indicate that climatic changes affect both the flux and composition of weathered material delivered to the oceans on glacial-interglacial timescales.

Geophysical Research Letters, 1998

The first oxygen isotope analysis of biogenic opal from lake sediments, from the Aller6d/Younger Dryas transition in a core from Linsley Pond, Connecticut, gives an average estimate of a 6øC drop in temperature during the Younger Dryas. This shift represents temperatures during the bloom season, and may be less than the winter temperature drop. The sharp transition itself, with a duration of about 200 years, suggests that the temperature decrease may have been as large as 12øC. Previous estimates of the Aller6d/Younger Dryas temperature shifts are controversial, and range from 3-20øC, suggesting that further interdisciplinary research on the same samples is warranted. One way that global climate change manifests itself is by redistributing energy throughout the globe. The Northern Hemisphere latitudinal temperature gradient during the lateglacial is at present a controversial topic. The magnitude of air temperature shifts during the Aller6d/Younger Dryas (YD) oscillation are estimated from mid-latitude pollen records surrounding the North Atlantic to be 3-5øC in Europe [Lowe et al., 1994] and 3-4øC in the eastern US [Peteet et al., 1993]. In contrast, lake temperatures estimates derived from aquatic midge larvae in the Canadian eastern maritimes and Maine range from 6-20øC, with larger shifts at more southern sites [Levesque et al., 1997]. The magnitude of YD cooling in Greenland ice cores ranges from at least 7øC from the B611ing warming [Dansgaard et al., 1989] to 15øC-a more recent estimate from borehole temperatures [Cuffey et al., 1995]. The ice core geochemical records reveal that massive frequent and short-term (decadal or less) changes in atmospheric composition occurred throughout this event, suggesting a very dynamic circulation [Mayewski et al., 1993]. To date, studies of oxygen isotopes in lake sediments have been restricted to the carbonate phase [McKenzie and Hollander, 1993], analyzing authigenic carbonates and calcareous bottom dwellers such as ostracods. Previous isotopic estimates of climate change from North America carbonates in lakes are few [Stuiver, 1970; Yu and Eicher, 1995] and although a late-glacial shift is apparent, it is undated. In Europe, the oxygen isotope estimates in the Swiss lakes range from 1-3 permil and were interpreted as a change in moisture source and air temperature [Eicher and Siegenthaler, 1976; Siegenthaler et al., 1984]. Because of the uncertainty in defining the moisture sources and transport pathways, the estimates of temperature change are also variable. Carbonate material suitable for isotopic analysis is

Journal of Paleolimnology, 2005

Comparisons between climate proxies and instrumental records through the last two centuries are often used to understand better the controls on palaeoarchives and to find relationships that can be used to quantify changes in pre-instrumental climate. Here we compare an 80-year-long annually resolved oxygen isotope record from Nar Gölü, Turkey, a varved lake sequence, with instrumental records of temperature, precipitation, wind speed, relative humidity and calculated values of evaporation, all of which are known to be possible controls on lake oxygen isotope systems. Significant relationships are found between the isotope record and summer temperatures and evaporation suggesting these are dominant controls on the isotope hydrology of this non-outlet lake. Modelling the stable isotope hydrology of the lake system allows these relationships to be tested independently. We show that the isotope record follows the same trends in the temperature and evaporation records but that, even when combined, these two climatic factors cannot fully explain the magnitude of change observed in the isotope record. The models show the lake system is much less sensitive to changes in evaporation and temperature than the climate calibration suggests. Additional factors, including changes in the amount of precipitation, are required to amplify the isotope change. It is concluded that proxy-climate calibrations may incorrectly estimate the amplitude of past changes in individual climate parameters, unless validated independently.

Quaternary Science Reviews, 2004

The isotope composition of authigenic and biogenic carbonates and diatom silica are commonly used as palaeoclimate proxies from lake sediments. This article reviews the controls on the isotope composition of lacustrine skeletal and non skeletal deposits and illustrates how stable isotope studies contribute to an understanding of changes in temperature, precipitation patterns, evaporation and the carbon cycle. It highlights the differences in the palaeoclimate potential of a wide range of lakes ranging from open to closed lake basins. A large number of the case histories, but not all, are drawn from studies of temperate lakes from Europe. Large closed lake systems, in the tropics and elsewhere, lose water predominantly through evaporation, and contain sediments with variable and generally high δ 18 O values. Fluctuations in the isotope composition of authigenic or biogenic minerals are mainly a function of long term changes in the precipitation/evaporation ratio. In contrast small open lakes which have a degree of through-flow typically contain sediments with δ 18 O values that vary by no more than a few ‰. These variations are generally ascribed to variations in temperature or the isotope composition of precipitation (δp), from which either an annual or seasonally specific signal can be gained. These types of lakes are common in Northern Europe and at high altitudes. The interpretation of isotope data from a lacustrine succession requires a knowledge of the local processes that might control and modify the signal. Their effects need to be quantified, and a robust calibration using the modern lake system is necessary to establish the relationship between the measured signal, the isotopic composition of the host waters, and climate.

We determined oxygen isotope ratios (δ 18 O) of the mixed layerdwelling planktonic foraminifer Globigerina bulloides and relative abundance of Neogloboquadrina pachyderma (sinistral) for the last glacial-interglacial cycle, including marine isotope Stage (MIS) 6 from Integrated Ocean Drilling Program Site U1313. The δ 18 O data in G. bulloides allowed us to construct an age model. Changes in relative abundance of N. pachyderma (s) were compared to the δ 18 O of G. bulloides. We also performed a detailed quantitative study of four planktonic foraminiferal species to assess surface to near-surface oceanographic changes for the period between MIS 5e and 6 (120-180 ka), including the penultimate glacial termination (TII). Analyses of the relative abundances of four planktonic foraminiferal species lead us to reconstruct the variability in the upper water masses of the North Atlantic at this subtropical site. We found clear indications of freshwater inputs, including a Heinrich iceberg-rafting event (H11), which provided a mechanism for freshening the sea surface and perturbing climate conditions.