Deep Atlantic Ocean carbon storage and the rise of 100,000-year glacial cycles

Abstract

Over the past three million years, Earth's climate oscillated between warmer interglacials with reduced terrestrial ice volume and cooler glacials with expanded polar ice sheets. These climate cycles, as reflected in benthic foraminiferal oxygen isotopes, transitioned from dominantly 41-kyr to 100-kyr periodicities during the mid-Pleistocene (1,250 to 700 ka). Because orbital forcing did not shift at this time, the ultimate cause of this mid-Pleistocene transition (MPT) remains enigmatic. Here we present foraminiferal trace element (B/Ca, Cd/Ca) and Nd isotope data that demonstrate a tight linkage between Atlantic Ocean meridional overturning circulation and deep-ocean carbon storage across the MPT. Specifically, between 950 and 900 ka, carbonate ion saturation decreased by 30 µmol/kg and phosphate concentration increased by 0.5 µmol/kg coincident with a 20% reduction of North Atlantic Deep Water contribution to the abyssal South Atlantic. These results demonstrate that the glacial deep Atlantic carbon inventory increased by approximately 50 gigatons during the transition to 100-kyr glacial cycles. We suggest that the coincidence of our observations with evidence for increased terrestrial ice volume reflects how weaker overturning circulation and Southern Ocean biogeochemical feedbacks facilitated deep ocean carbon storage, which lowered atmospheric pCO 2 and thereby enabled expanded terrestrial ice volume at the MPT. Cyclic glaciations are the primary feature of Earth's climate since the late Pliocene and occur at periodicities linked to variations in solar insolation 1. However, the dominant periodicity of glaciations transitioned from 41-kyr to 100-kyr during the mid-Pleistocene without concomitant changes in external insolation forcing 2-5. It has been

Plio‐Pleistocene Hemispheric (A)Symmetries in the Northern and Southern Hemisphere Midlatitudes

“…Furthermore, all of the climate records we analyze here attain coherence (above 80% confidence interval) with Site 1090 iron ( Figure S7b) and dust accumulation rates (not shown) between 1.6 and 2.0 Ma, lending additional support for this proposed mechanism. Though our findings do not rule out additional carbon cycle responses in conjunction with ocean circulation changes (Farmer et al, 2019;Lear et al, 2016;Pena & Goldstein, 2014) or enhanced silicate weathering following regolith removal (Clark et al, 2006), such changes have been proposed to occur much later in the MPT (between 0.7 and 0.9 Ma), whereas here we infer the strengthening of orbital-scale climate feedbacks before the MPT.…”

“…Heightened sensitivity to obliquity forcing in a suite of orbitally resolved SST records from multiple climatic and oceanographic settings as well as in benthic ∂ 18 O is most easily explained by a global greenhouse gas forcing mechanism. Multiple studies have invoked a carbon cycle change to explain the origin of the MPT, for example, via iron fertilization in the Southern Ocean (Martínez-Garcia et al, 2011), ocean overturning circulation changes leading to an increase in deep sea carbon storage (Farmer et al, 2019;Lear et al, 2016;Pena & Goldstein, 2014) or enhanced silicate weathering (Clark et al, 2006). Such a carbon cycle change could have operated with or without the additional influence of a change in ice sheet dynamics, by either regolith removal (Clark & Pollard, 1998) or phase locking of the Northern and Southern Hemisphere ice sheets (Raymo et al, 2006), though recent modeling results support the interpretation that a carbon cycle change acted in tandem with evolving ice sheet dynamics in order to trigger the transition to the 100-kyr world (Chalk et al, 2017;Willeit et al, 2019).…”

“…A second major climate shift, the mid-Pleistocene transition (MPT;~0.7-1.25 Ma), is defined by what appears to be a globally synchronous switch from dominant 41-to~100-kyr climate cycles, for which the ultimate cause is still unknown (e.g., Chalk et al, 2017;Clark & Pollard, 1998;Farmer et al, 2019;Pena & Goldstein, 2014;Raymo et al, 2006;Willeit et al, 2019). Secular-scale and orbital-scale changes in greenhouse gas concentrations have been invoked to explain both the LPT (e.g., Lunt et al, 2008;Martínez-Botí et al, 2015;Seki et al, 2010) and the MPT (Chalk et al, 2017;Farmer et al, 2019;Hönisch et al, 2009;Willeit et al, 2019), the latter of which may have also involved critical changes in ice sheet dynamics in either the Northern (Clark & Pollard, 1998) or Southern (Raymo et al, 2006) Hemisphere cryosphere. Given important questions regarding the hemispheric symmetry of climate evolution over these profound transitions, as well as the critical role of the Southern Hemisphere in carbon cycling, better characterization of Southern Hemisphere climate evolution over the Plio-Pleistocene is essential in improving our understanding of the mechanisms behind the significant climate changes that occurred over this interval.…”

Data constraints on ocean-carbon cycle feedbacks at the mid-Pleistocene transition

“…Translated to total dissolved carbon, these observations support a ~50 µmol/kg increase during glacials after 950 kyr bP (Fig. 2d), equivalent to a 50 billion ton increase in carbon inventory throughout the deep Atlantic (Farmer et al 2019). Pairing this information with ε Nd , Farmer et al (2019) demonstrated that this deep-ocean carbon accumulation coincided with weakened deep Atlantic Ocean circulation ( Fig.…”

“…2d), equivalent to a 50 billion ton increase in carbon inventory throughout the deep Atlantic (Farmer et al 2019). Pairing this information with ε Nd , Farmer et al (2019) demonstrated that this deep-ocean carbon accumulation coincided with weakened deep Atlantic Ocean circulation ( Fig. 2c-d), suggesting that weaker circulation facilitated accumulation of carbon and other nutrients in the deep Atlantic.…”