Carbon isotopes characterize rapid changes in atmospheric carbon dioxide during the last deglaciation

Abstract

An understanding of the mechanisms that control CO 2 change during glacial-interglacial cycles remains elusive. Here we help to constrain changing sources with a high-precision, high-resolution deglacial record of the stable isotopic composition of carbon in CO 2 (δ 13 C-CO 2 ) in air extracted from ice samples from Taylor Glacier, Antarctica. During the initial rise in atmospheric CO 2 from 17.6 to 15.5 ka, these data demarcate a decrease in δ 13 C-CO 2 , likely due to a weakened oceanic biological pump. From 15.5 to 11.5 ka, the continued atmospheric CO 2 rise of 40 ppm is associated with small changes in δ 13 C-CO 2 , consistent with a nearly equal contribution from a further weakening of the biological pump and rising ocean temperature. These two trends, related to marine sources, are punctuated at 16.3 and 12.9 ka with abrupt, century-scale perturbations in δ 13 C-CO 2 that suggest rapid oxidation of organic land carbon or enhanced air-sea gas exchange in the Southern Ocean. Additional century-scale increases in atmospheric CO 2 coincident with increases in atmospheric CH 4 and Northern Hemisphere temperature at the onset of the Bølling (14.6-14.3 ka) and Holocene (11.6-11.4 ka) intervals are associated with small changes in δ 13 C-CO 2 , suggesting a combination of sources that included rising surface ocean temperature.ice cores | paleoclimate | carbon cycle | atmospheric CO 2 | last deglaciation O ver thirty years ago ice cores provided the first clear evidence that atmospheric CO 2 increased by about 75 ppm as Earth transitioned from a glacial to an interglacial state (1, 2). After decades of research, the underlying mechanisms that drive glacial-interglacial CO 2 cycles are still unclear. A tentative consensus has formed that the deglaciation is characterized by a net transfer of carbon from the ocean to the atmosphere and terrestrial biosphere, through a combination of changes in ocean temperature, nutrient utilization, circulation, and alkalinity. Partitioning these changes in terms of magnitude and timing is challenging. Estimates of the glacial-interglacial carbon cycle budget are highly uncertain, ranging from 20-30 ppm for the effect of rising ocean temperature, 5-55 ppm for ocean circulation changes, and 5-30 ppm for decreasing iron fertilization (3, 4), with feedbacks from CaCO 3 compensation accounting for up to 30 ppm (5, 6).A precise history of the stable isotopic composition of atmospheric carbon dioxide (δ 13 C-CO 2 ) can constrain key processes controlling atmospheric CO 2 (7,8). A low-resolution record from the Taylor Dome ice core (9) identified a decrease in δ 13 C-CO 2 at the onset of the deglacial CO 2 rise that was followed by increases in both CO 2 and δ 13 C-CO 2 (Fig. 1). A higher-resolution record from the European Project for Ice Coring in Antarctica Dome C (EDC) ice core (10) provided additional support for the rapid δ 13 C-CO 2 decrease associated with the initial CO 2 rise, and box modeling indicated that this decrease was consistent with changes in marine productivity. The r...

Atmospheric gas records from Taylor Glacier, Antarctica, reveal ancient ice with ages spanning the entire last glacial cycle

“…This hypothesis is corroborated by satellite imagery of the area which appears devoid of stratigraphic features seen in other locations on the glacier, and a deep vertical core that found horizontal upright stratigraphy using gases. Finally, the N 2 O and CO 2 records from Schilt et al (2014) and Bauska et al (2016), respectively, confirm that the section spanning −124 to +120 m on our sampling line (Fig. 3c) represents the last deglaciation, in excellent agreement with our δ 18 O atm and methane data.…”

“…At this site virtually unlimited amounts of old ice can be collected at the surface, allowing for the application of new proxies that have hitherto been precluded by sample size restrictions Petrenko et al, 2016), and increasing the precision of established measurements that are hindered by small sample volumes. Two studies investigating the isotopic composition of N 2 O and CO 2 covering the deglaciation from Taylor Glacier are already published (Schilt et al, 2014;Bauska et al, 2016). Here, we provide the detailed age model for these studies and expand the temporal framework from the deglaciation to the entire last glacial cycle.…”

Reconstructing deglacial ocean ventilation using radiocarbon : data and inverse modeling

“…A rapid southward shift of Southern Hemisphere westerlies and upwelling is also consistent with the reported southward shift of the intertropical convergence zone within HS1 (Broecker and Putnam, 2012;Peterson et al, 2000). This shift of atmospheric circulation regime was accompanied by further drying in the Northern (Wang et al, 2001) and wetting in the Southern Hemisphere (Wang et al, 2004), which are argued to be responses to iceberg discharge in the North Atlantic and contribute to a rapid CO2 increase and a CH4 spike at ~16.2 ka (Bauska et al, 2016;Rhodes et al, 2015). However, the rapid southward shift of the Southern Ocean upwelling could also contribute to the abrupt CO2 rise (as discussed in the main text).…”

Synchronous volcanic eruptions and abrupt climate change ∼17.7 ka plausibly linked by stratospheric ozone depletion

“…The result was a pronounced, synchronous, ∼50% decline in SH dust deposition (Fig. 1), reducing ocean biological uptake (10,46) and thereby sharply reducing the ocean CO 2 sink. In Patagonia and New Zealand, warmer and dryer conditions south of ∼35°S (37) led to the well-documented retreat of glaciers (4-6) that starved glacial outwash plains of their fine-sediment resupply (45), with lower wind speeds possibly also contributing to reduced dustiness (47).…”