Marine Reservoir Age Variability Over the Last Deglaciation: Implications for Marine CarbonCycling and Prospects for Regional Radiocarbon Calibrations

Abstract

Marine radiocarbon dates, corrected for ocean-atmosphere reservoir age offsets (R-ages), are widely used to constrain marine chronologies. Rages also represent the surface boundary condition that links the ocean interior radiocarbon distribution (i.e., "radiocarbon ventilation ages") to the ocean's large-scale overturning circulation. Understanding how Rages have varied over time is therefore essential both for accurate dating and for investigations into past ocean circulation/carbon cycle interactions. A number or recent studies have shed light on surface reservoir age changes over the last deglaciation; however, a clear picture of global/regional spatiotemporal patterns of variability has yet to emerge. Here we combine new and existing reservoir age estimates to show coherent but distinct regional reservoir age trends in the subpolar North Atlantic and Southern Ocean. It can be further shown that similar but lower amplitude changes occurred at midlatitudes in each hemisphere. An apparent link between regional patterns of reservoir age variability and the "thermal bipolar seesaw" suggests a causal link with changes in ocean circulation, mixed-layer depth, and/or sea ice dynamics. A further link to atmospheric CO 2 is also apparent and underlines a potentially dominant role for changes in the ocean's "disequilibrium carbon" pool, rather than changes in ocean transport, in deglacial CO 2 change. The existence of significant Rage variability over the last deglaciation poses a problem for marine radiocarbon age calibrations. However, its apparent regional consistency also raises the prospect of developing region-specific marine calibration curves for radiocarbon-dating purposes. Plain Language Summary Radiocarbon is widely used to date ancient fossil material, including marine shells, reaching back to~40,000 years. Less well known, is its use as a marine carbon cycle tracer. Both of these applications require knowledge of how the surface ocean's radiocarbon activity has changed over time, which presents a serious challenge. In this study, we demonstrate that the polar regions of the Atlantic Ocean have experienced significant changes in their radiocarbon activity. These are linked to both regional climate change and atmospheric CO 2 fluctuations, and thus serve to emphasize the important role of processes acting at the sea surface, including sea ice variability in particular, in controlling the heat and carbon storage in the ocean. At the same time, by demonstrating regional consistency of marine radiocarbon trends, our results open up the possibility of improved radiocarbon dating of marine material in future.

Southern Ocean convection amplified past Antarctic warming and atmospheric CO2 rise during Heinrich Stadial 4

“…periods of reduced AMOC 14 ) has further implications for atmospheric CO 2 , as well as global energy balance on millennial time scales. Our findings support the proposal that deep convection in the Southern Ocean helped to amplify atmospheric CO 2 change during North Atlantic stadials 12 , and that a loss of respired and/or 'disequilibrium' carbon from the deep Southern Ocean during North Atlantic stadials 37 was driven at least in part by changes in air-sea gas exchange (as distinct from changes in vertical mass overturning rates or the export of biologically fixed carbon for example) 41 . Finally, it has not escaped our attention that the occurrence of enhanced Southern Ocean convection during periods of reduced AMOC is also likely to bear on the evolution of global ocean heat content across thermal bipolar seesaw events, with further implications for global average surface air temperatures and 'climate sensitivity' on millennial time scales.…”

Glacial heterogeneity in Southern Ocean carbon storage abated by fast South Indian deglacial carbon release

“…Second, glacial d 14 R P-Atm estimates in South Atlantic core MD07-3076CQ reach values larger than 2000 years during the LGM 5 , which may be unrealistic according to a new compilation 48 . Disregarding these extreme d 14 R P-Atm values (following similar sensitivity tests made by ref.…”

Marine20—The Marine Radiocarbon Age Calibration Curve (0–55,000 cal BP)

“…Firstly, one could aim to obtain sufficient data from individual locations in order to construct regional empirical curves. This would require considerable data, and it may be difficult to achieve sufficiently global coverage, but such work has begun in the Atlantic (Muschitiello et al 2019;Brendryen et al 2019;Skinner et al 2019;Waelbroeck et al 2019;Burke et al submitted) and globally (e.g. Sarnthein et al 2015).…”