Sequential changes in ocean circulation and biological export productivity during the last glacial cycle: a model-data study

Abstract

We conduct a model-data analysis of the ocean, atmosphere and terrestrial carbon system to understand their effects on atmospheric CO 2 during the last glacial cycle. We use a carbon cycle box model "SCP-M", combined with multiple proxy data for the atmosphere and ocean, to test for variations in ocean circulation and biological productivity across marine isotope stages spanning 130 thousand years ago to the present. The model is constrained by proxy data associated with a range of 5 environmental conditions including sea surface temperature, salinity, ocean volume, sea ice cover and shallow water carbonate production. Model parameters for global ocean circulation, Atlantic meridional overturning circulation and Southern Ocean biological export productivity are optimised in each marine isotope stage, against proxy data for atmospheric CO 2 , δ 13 C and ∆ 14 C and deep ocean δ 13 C, ∆ 14 C and carbonate ion. Our model-data results suggest that global overturning circulation weakened at marine isotope stage 5d, coincident with a ∼25 ppm fall in atmospheric CO 2 from the penultimate interglacial 10 level. This change was followed by a further slowdown in Atlantic meridional overturning circulation and enhanced Southern Ocean biological export productivity at marine isotope stage 4 (∼-30 ppm). There was also a transient slowdown in Atlantic meridional overturning circulation at MIS 5b. In this model, the last glacial maximum was characterised by relatively weak global ocean and Atlantic meridional overturning circulation, and increased Southern Ocean biological export productivity (∼-20 ppm during MIS 2-4). Ocean circulation and Southern Ocean biology rebounded to modern values by the Holocene period. 15The terrestrial biosphere decreased by ∼500 Pg C in the lead up to the last glacial maximum, followed by a period of intense regrowth during the Holocene (∼750 Pg C). Slowing ocean circulation, a cooler ocean and, to a lesser extent, shallow carbonate dissolution, contributed ∼-75 ppm to atmospheric CO 2 in the ∼100 thousand-year lead-up to the last glacial maximum, with a further ∼-10 ppm contributed during the glacial maximum. Our model results also suggest that an increase in Southern Ocean biological productivity was one of the ingredients required to achieve the last glacial maximum atmospheric CO 2 level. The 20 incorporation of longer-timescale data into quantitative ocean transport models, provides useful insights into the timing of changes in ocean processes, enhancing our understanding of the last glacial maximum and Holocene carbon cycle transition.Large and regular fluctuations in atmospheric CO 2 and ocean proxy signals for carbon isotopes and carbonate ion concentration, over the last 800 kyr, are preserved in ice and marine core records. The most obvious of these fluctuations is the repeated oscillation of atmospheric CO 2 over the range of ∼180-280 ppm every ∼100 kyr. The magnitude and regularity of these oscillations in atmospheric CO 2 , combined with proxy observations for carbon isotopes, point to ...