Water Column Stratification in the Antarctic Zone of the Southern Ocean During the Mid‐Pleistocene Climate Transition

Abstract

We use biogenic silica (opal) mass accumulating rates (MARs) at Ocean Drilling Program Site 745B to infer upwelling, and by extension upper water column stratification, on glacial to interglacial time scales during the mid-Pleistocene climate transition (MPT; 1.2-0.6 Ma). Distinct variations in the percent biogenic silica content of the sediments parallel the global benthic foraminiferal δ 18 O stack affording the derivation of an orbital-scale age model. Opal MARs confirm that silica production/preservation was indeed at a minimum at each of the glacial maxima during the entire MPT consistent with the late Pleistocene model that links upwelling of dissolved silica with water column stratification. A unique relationship between opal MARs and benthic foraminiferal δ 13 C values in the South Atlantic suggests that during the midpoint of the MPT stratification may also be causally related to deep water circulation. However, there is no evidence that cooling during the MPT and the evolution of the 100 kyr climate cycle are linked to changes in water column stratification. Terrigenous sediment MARs, on the other hand, which can be inferred from the proportion of biogenic silica in the sediments at this particular site, provide evidence for increased glacial activity on Antarctica beginning with the onset of the MPT and culminating in a maximum during its midpoint (Marine Isotope Stage 22, 0.9 Ma). Thereafter, glacial activity decreases and we speculate that the evolution of the 100 kyr cycle is associated with a thickening ice sheet after reaching maximum extent somewhat analogous to the processes proposed for Northern Hemisphere ice sheets.

A 1 Million Year Record of Biogenic Silica in the Indian Ocean Sector of the Southern Ocean: Regional Versus Global Forcing of Primary Productivity

“…Until MIS 12, maxima in the percent opal are easily related to pronounced interglacial climate conditions. As pointed out by Billups et al (2018), during the mid-Pleistocene (MIS 31-16), opal percentages essentially mimic the δ 18 O record. The new data indicate that this characteristic pattern extends through the MIS 12-11 transition and across the last glacial to interglacial cycle (MIS 4-1).…”

Geochemical Records of the Provenance and Silicate Weathering/Erosion From the Eastern Arabian Sea and Their Responses to the Indian Summer Monsoon Since the Mid‐Pleistocene

“…This variation pattern in the Indian summer monsoon coincides well with a major increase in monsoon‐related productivity in the tropical Indian Ocean (Gupta et al, 2008), increased moisture variability on the Tibetan Plateau (Koutsodendris et al, 2018), and pronounced variability in hydrochemical conditions in the Co Ngoin area in Tibet (Jin et al, 2011) during the MPT. The intensification of the Indian summer monsoon might have been linked to increased cross‐equatorial pressure gradient between the Mascarene high (over the southern subtropical Indian Ocean) and the Indian low (over the Asian continent) (An et al, 2011; Clemens et al, 1996) as a result of the expanded Antarctic ice sheets (Billups et al, 2018; Elderfield et al, 2012; Farmer et al, 2019), which may have led to a strong Mascarene high. The significant 29‐kyr cycle observed in α Al K during ~1.2–0.7 Ma further supports the strong sensitivity of the Indian summer monsoon to Southern Hemispheric processes during the MPT (Clemens & Prell, 1991; Figure 8d).…”

Closure of the Bering Strait caused Mid-Pleistocene Transition cooling

“…Southern Ocean productivity proxies are currently of low resolution 15 , and although they show consistently increased glacial water column stratification, thought critical for sequestration of glacial CO 2 (ref. 14 ), they do not show a clear increase in stratification over the MPT 15 highlighting the potential importance of NPIW in increased oceanic sequestration of CO 2 during the MPT. Although a further enhancement of dust deposition across the Southern Ocean may have contributed to MIS 22–24 cooling and CO 2 decline (Fig.…”

“…Glacial reductions in Bering Sea Green Belt primary productivity are similar both to changes in subarctic Pacific primary productivity (ODP Site 882) over at least the last 800 ka 16,21,24,25 , and to glacial reductions in productivity in the Indian Ocean sector of the Southern Ocean (ODP Site 745) 15 , although the causes of reduced productivity are yet to be identified. For example, reductions in Pacific glacial productivity may be caused by light limitation due to a deepening of the mixed layer during summer 24 , hypothetically linked to changing atmospheric weather systems and windiness.…”

“…These processes are not mutually exclusive, and could each be associated with a build-up of ice on North America for which there is ample evidence 4 . To sequester CO 2 in the deep ocean also likely requires stifled nutrient and CO 2 -rich deep water upwelling at high latitudes to reduce outgassing, as proposed for the last glacial; 13,14 however, although recent records of Southern Ocean biogenic opal suggest glacial reductions in productivity occurred throughout the Pleistocene 15 , there are so far no direct records of high latitude upwelling from either hemisphere with which to evaluate this potentially important mechanism for the MPT glacial CO 2 reduction.…”