Two contributors to the glacial CO 2 decline

Past Carbonate Preservation Events in the Deep Southeast Atlantic Ocean (Cape Basin) and Their Implications for Atlantic Overturning Dynamics and Marine Carbon Cycling

“…Moreover, the water column density structure and vertical mixing in the Southern Ocean have varied over glacial‐interglacial cycles (Hodell et al, ; Roberts et al, ), which may have modulated the whole‐ocean mass balance response to perturbations of the ocean carbonate chemistry system or caused changes in carbonate preservation directly. Interestingly, it would appear that carbonate preservation changes in the deep Cape Basin are not dominated by major changes in volcanic CO 2 outgassing across glacial cycles (Broecker et al, ), as this would be expected to cause enhanced preservation during glacial inceptions and enhanced dissolution during deglaciations.…”

“…However, other processes that are broadly synchronous with δ 18 O w may also have triggered whole-ocean mass balance adjustments of the carbonate system or may have contributed to carbonate dissolution directly, for example, changes in the CaCO 3 -to-organic carbon ratio of exported detritus (Archer & Maier-Reimer, 1994), the waxing and waning of the terrestrial biosphere (Ridgwell et al, 2003), changes in the carbonate system dissociation constants owing to hydrostatic pressure, deep ocean temperature, and salinity variations (Kohfeld & Ridgwell, 2009;Ridgwell, 2001), changes in deep ocean carbon storage (Boyle, 1988b;Kerr et al, 2017) and ventilation (Tschumi et al, 2011), interbasin deep water and alkalinity exchange (Sosdian et al, 2018;Yu et al, 2014), or planetary outgassing of CO 2 (Broecker et al, 2015;Huybers & Langmuir, 2009). For instance, the good correspondence of bottom water temperatures at ODP Site 1123 and the low-frequency XRF Ca/Ti record (although maximum cross correlation is observed at lag = 24.9 kyr over the last 400 kyr; Figure 7) would may suggest that some fraction of the orbital-scale carbonate variability may result from changes in hydrographic water mass properties, impacting on carbonate mineral solubility and ocean CO 2 solubility and possibly leading to carbonate compensation.…”

The Evolution of Deep Ocean Chemistry and Respired Carbon in the Eastern Equatorial Pacific Over the Last Deglaciation

“…Deep water [CO 3 2− ] offset between LGM (~18–23 kyr) and Holocene/modern (~0–8 kyr) from several Atlantic/Southern Ocean (in blue) and Indo‐Pacific Ocean (in orange) sediment cores. DSDP593Z (Elmore et al, ); RR0503‐83 (Allen et al, ); MW91‐9 GGC15, MW91‐9 GGC48, WIND 28K, and VM28‐122 (Yu et al, ); TTNO13 PC61 (Yu et al, ); BOFS and NEAP cores (Yu et al, ; Yu & Elderfield, ); RC13‐140, RC23‐22, and RC23‐15 (Doss & Marchitto, ); RC16‐59 (Broecker et al, ); GeoB cores (Raitzsch et al, ); MD07‐3076 (Gottschalk et al, ); TN057‐21 (Yu, Anderson, Jin, et al, ); and ODP1240 (this study, highlighted in bold). WP: western Pacific; EEP: Eastern Equatorial Pacific; CP: central Pacific; NEA: North East Atlantic; WEA: western equatorial Pacific; EqA: equatorial Atlantic; SO: Southern Ocean; IO: Indian Ocean.…”

“…If a whole ocean change of ~0.3‰ (Peterson et al, ) is assumed to have occurred exclusively due to terrestrial carbon release (Shackleton, ), a total change of ~0.56‰ would therefore be expected, which is not very far off the results of Doss and Marchitto () and our observed glacial‐interglacial δ 13 C difference of 0.4 ± 0.06‰. However, it is notable that the observed global average glacial‐interglacial δ 13 C change of ~0.3‰ corresponds to a global average radiocarbon ventilation age change of perhaps ~689 14 C yr (Skinner et al, ), which raises the question of how this average change can be partitioned into a “terrestrial carbon component” and a “ventilation/respired carbon component.” Until this issue is resolved, along with the related issue of how subaerial volcanism changed across the last glacial period (Broecker et al, ), the possibility remains that much of the full glacial‐interglacial δ 13 C difference observed in ODP1240 was achieved by respired carbon addition, implying that a bigger drop in [CO 3 2− ] would be expected. This, in turn, would point to an additional process that would have counterbalanced the addition of [CO 3 2− ] from organic carbon respiration, such as CaCO 3 dissolution.…”

CO2 Release From Pockmarks on the Chatham Rise‐Bounty Trough at the Glacial Termination

“…Instead, these excursions are hypothesized to have come from release of geologic carbon from hydrothermal systems nearby (Stott and Timmermann, 2011;Stott et al, 2019), possibly released when temperatures rose and destabilized the CO2 reservoirs. Other studies have considered ways for geologic systems to influence carbon cycling on glacial time scales (Broecker et al, 2015;Huybers and Langmuir, 2017;Lund and Asimow, 2011;Ronge et al, 2016). But testing any of these hypotheses is difficult because the observational database is sparse, and the radiocarbon anomalies may involve more than one process.…”