Snapshots of mean ocean temperature over the last 700 000 years using noble gases in the EPICA Dome C ice core

Together with the latent heat stored in glacial ice sheets, the ocean heat uptake carries the lion’s share of glacial–interglacial changes in the planetary heat content, but little direct information on the global mean ocean temperature (MOT) is available to constrain the ocean temperature response to glacial–interglacial climate perturbations. Using ratios of noble gases and molecular nitrogen trapped in the Antarctic EPICA Dome C ice core, we are able to reconstruct MOT for peak glacial and interglacial conditions during the last 700 000 years and explore the differences between these extrema. To this end, we have to correct the noble gas ratios for gas transport effects in the firn column and gas loss fractionation processes of the samples after ice core retrieval using the full elemental matrix of N2, Ar, Kr, and Xe in the ice and their individual isotopic ratios. The reconstructed MOT in peak glacials is consistently about 3.3+-0.4°C cooler compared to the Holocene. Lukewarm interglacials before the Mid-Brunhes Event 450 kyr ago are characterized by 1.6+-0.4°C lower MOT than the Holocene; thus, glacial–interglacial amplitudes were only about 50%of those after the Mid-Brunhes Event, in line with the reduced radiative forcing by lower greenhouse gas concentrations and their Earth system feedbacks. Moreover, we find significantly increased MOTs at the onset of Marine Isotope Stage 5.5 and 9.3, which are coeval with CO2 and CH4 overshoots at that time.We link these CO2 and CH4 overshoots to a resumption of the Atlantic Meridional Overturning Circulation, which is also the starting point of the release of heat previously accumulated in the ocean during times of reduced overturning

Controls on Millennial‐Scale Atmospheric CO2 Variability During the Last Glacial Period

Changes in atmospheric CO2 on millennial‐to‐centennial timescales are key components of past climate variability during the last glacial and deglacial periods (70‐10ka) yet the sources and mechanisms responsible for the CO2 fluctuations remain largely obscure. Here we report the 13C/12C ratio of atmospheric CO2 during a key interval of the last glacial period at sub‐millennial resolution, with coeval histories of atmospheric CO2, CH4 and N2O concentrations. The carbon isotope data suggest that the millennial‐scale CO2 variability in MIS3 is driven largely by changes in the organic carbon cycle, most likely by sequestration of respired carbon in the deep ocean. Centennial‐scale CO2 variations, distinguished by carbon isotope signatures, are associated with both abrupt hydrological change in the tropics (e.g. Heinrich Events) and rapid increases in northern hemisphere temperature (DO events). These events can be linked to modes of variability during the last deglaciation, thus suggesting that drivers of millennial and centennial CO2 variability during both periods are intimately linked to abrupt climate variability.National Science Foundatio

Does δ18O of O2 record meridional shifts in tropical rainfall?

Marine sediments, speleothems, paleo-lake elevations, and ice core methane and δ¹⁸O of O₂ (δ¹⁸Oatm) records provide ample evidence for repeated abrupt meridional shifts in tropical rainfall belts throughout the last glacial cycle. To improve understanding of the impact of abrupt events on the global terrestrial biosphere, we present composite records of δ¹⁸Oatm and inferred changes in fractionation by the global terrestrial biosphere (ΔεLAND) from discrete gas measurements in the WAIS Divide (WD) and Siple Dome (SD) Antarctic ice cores. On the common WD timescale, it is evident that maxima in ΔεLAND are synchronous with or shortly follow small-amplitude WD CH₄ peaks that occur within Heinrich stadials 1, 2, 4, and 5 – periods of low atmospheric CH₄ concentrations. These local CH₄ maxima have been suggested as markers of abrupt climate responses to Heinrich events. Based on our analysis of the modern seasonal cycle of gross primary productivity (GPP)-weighted δ¹⁸O of terrestrial precipitation (the source water for atmospheric O₂ production), we propose a simple mechanism by which ΔεLAND tracks the centroid latitude of terrestrial oxygen production. As intense rainfall and oxygen production migrate northward, ΔεLAND should decrease due to the underlying meridional gradient in rainfall δ¹⁸O. A southward shift should increase ΔεLAND. Monsoon intensity also influences δ¹⁸O of precipitation, and although we cannot determine the relative contributions of the two mechanisms, both act in the same direction. Therefore, we suggest that abrupt increases in ΔεLAND unambiguously imply a southward shift of tropical rainfall. The exact magnitude of this shift, however, remains under-constrained by ΔεLAND

A horizontal ice core from Taylor Glacier, its implications for Antarctic climate history, and an improved Taylor Dome ice core time scale

Ice core records from Antarctica show mostly synchronous temperature variations during the last deglacial transition, an indication that the climate of the entire continent reacted as one unit to the global changes. However, a record from the Taylor Dome ice core in the Ross Sea sector of East Antarctica has been suggested to show a rapid warming, similar in style and synchronous with the Oldest Dryas—Bølling warming in Greenland. Since publication of the Taylor Dome record, a number of lines of evidence have suggested that this interpretation is incorrect and reflects errors in the underlying time scale. The issues raised regarding the dating of Taylor Dome currently linger unresolved, and the original time scale remains the de facto chronology. We present new water isotope and chemistry data from nearby Taylor Glacier to resolve the confusion surrounding the Taylor Dome time scale. We find that the Taylor Glacier record is incompatible with the original interpretation of the Taylor Dome ice core, showing that the warming in the area was gradual and started at ∼18 ka BP (before 1950) as seen in other East Antarctic ice cores. We build a consistent, up‐to‐date Taylor Dome chronology from 0 to 60 ka BP by combining new and old age markers based on synchronization to other ice core records. The most notable feature of the new TD2015 time scale is a gas age—ice age difference of up to 12,000 years during the Last Glacial Maximum, by far the largest ever observed

Radiometric ⁸¹Kr dating identifies 120,000-year-old ice at Taylor Glacier, Antarctica

We present the first successful ⁸¹Kr-Kr radiometric dating of ancient polar ice. Krypton was extracted from the air bubbles in four ~350 kg polar ice samples from Taylor Glacier in the McMurdo Dry Valleys, Antarctica, and dated using Atom Trap Trace Analysis (ATTA). The ⁸¹Kr radiometric ages agree with independent age estimates obtained from stratigraphic dating techniques with a mean absolute age offset of 6 ± 2.5 ka. Our experimental methods and sampling strategy are validated by 1) ⁸⁵Kr and ³⁹Ar analyses that show the samples to be free of modern air contamination, and 2) air content measurements that show the ice did not experience gas loss. We estimate the error in the ⁸¹Kr ages due to past geomagnetic variability to be below 3 ka. We show that ice from the previous interglacial period (MIS 5e, 130-115 ka before present) can be found in abundance near the surface of Taylor Glacier. Our study paves the way for reliable radiometric dating of ancient ice in blue ice areas and margin sites where large samples are available, greatly enhancing their scientific value as archives of old ice and meteorites. At present, ATTA ⁸¹Kr analysis requires a 40-80 kg ice sample; as sample requirements continue to decrease ⁸¹Kr dating of ice cores is a future possibility.Keywords: paleoclimatology, glaciology, geochronolog

Spatial pattern of accumulation at Taylor Dome during the last glacial inception: stratigraphic constraints from Taylor Glacier

A new ice core retrieved from the Taylor Glacier blue ice area contains ice and air spanning the Marine Isotope Stage (MIS) 5/4 transition (74 to 65 ka), a period of global cooling and glacial inception. Dating the ice and air bubbles in the new ice core reveals an ice age-gas age difference (Δage) approaching 10 ka during MIS 4, implying very low accumulation at the Taylor Glacier accumulation zone on the northern flank of Taylor Dome. A revised chronology for the Taylor Dome ice core (80 to 55 ka), situated to the south of the Taylor Glacier accumulation zone, shows that Δage did not exceed 2.5 ka at that location. The difference in Δage between the new Taylor Glacier ice core and the Taylor Dome ice core implies a spatial gradient in snow accumulation across Taylor Dome that intensified during the last glacial inception and through MIS 4

Spatial pattern of accumulation at Taylor Dome during Marine Isotope Stage 4: stratigraphic constraints from Taylor Glacier

New ice cores retrieved from the Taylor Glacier (Antarctica) blue ice area contain ice and air spanning the Marine Isotope Stage (MIS) 5–4 transition, a period of global cooling and ice sheet expansion. We determine chronologies for the ice and air bubbles in the new ice cores by visually matching variations in gas- and ice-phase tracers to preexisting ice core records. The chronologies reveal an ice age–gas age difference (Δage) approaching 10 ka during MIS 4, implying very low snow accumulation in the Taylor Glacier accumulation zone. A revised chronology for the analogous section of the Taylor Dome ice core (84 to 55 ka), located to the south of the Taylor Glacier accumulation zone, shows that Δage did not exceed 3 ka. The difference in Δage between the two records during MIS 4 is similar in magnitude but opposite in direction to what is observed at the Last Glacial Maximum. This relationship implies that a spatial gradient in snow accumulation existed across the Taylor Dome region during MIS 4 that was oriented in the opposite direction of the accumulation gradient during the Last Glacial Maximum