Expansion of subarctic water masses in the North Atlantic and Pacific oceans and implications for mid‐Pleistocene ice sheet growth

Abstract

[1] Past surface ocean circulation changes associated with the mid-Pleistocene transition, 0.9-0.6 Ma, were reconstructed in the northern North Atlantic (ODP 983) and the northwest Pacific (ODP 882), using proxies for subarctic/subpolar water mass distributions (%C 37:4 alkenone) and sea surface temperature (U 37 K ). Both sites experienced a secular expansion of subarctic waters from $1.15 Ma, spanning both glacial and interglacial intervals. After 0.9 Ma, low %C 37:4 at Site 983 records a northward retreat of subarctic waters during interglacials in the Atlantic, while continued high glacial %C 37:4 indicate extensive subarctic waters during glacial maxima associated with the development of the larger late Pleistocene ice sheets. In contrast, a secular decline in %C 37:4 occurred at Site 882 from 0.9 to 0.5 Ma, marking a more gradual retreat of subarctic conditions in the Pacific. It is proposed that the expansion of subarctic waters between 1.15 and 0.9 Ma exerted negative feedbacks to the moisture supply to the ice sheet source regions and may account for the apparent delayed ice sheet response to atmosphere-ocean circulation changes associated with the mid-Pleistocene transition that began as early as 1.2 Ma.

A major change in North Atlantic deep water circulation 1.6 million years ago

“…Since pertinent records (e.g., Lawrence et al, 2009;Naafs et al, 2010) do not show any coeval decrease in poleward heat and salt transport within the NAC (present day ε Nd = −13; Lacan and Jeandel, 2004a, b), which would have resulted in diminished advection of "unradiogenic" subsurface waters into the Nordic Seas, the cooling was most likely linked to an expansion of subpolar waters (present day ε Nd = −10 to −8.5; Lacan and Jeandel, 2004a, b). Recent findings by Martínez-Garcia et al (2010) show, in fact, that during that very time the sub-Arctic region underwent a substantial cooling associated with sea-ice expansion, which continued across the mid-Pleistocene transition ∼ 1.15-0.9 Ma (McClymont et al, 2008). This major sub-Arctic cooling led to the formation and advection of cold deep waters from the GIN seas into the North Atlantic basin, which was recorded further south in the North Atlantic (ODP site 607) by a longterm decrease in NADW temperature after ∼ 1.55 Ma (Sosdian and Rosenthal, 2009), coeval with major surface water cooling in the area (see comparison in Lawrence et al, 2010).…”

Pleistocene sea-surface temperature evolution: Early cooling, delayed glacial intensification, and implications for the mid-Pleistocene climate transition

“…Whilst there is growing evidence for changing ocean/atmosphere circulation pre-dating, and accompanying, ice-sheet expansion (e.g. de Menocal, 1995;Heslop et al, 2002;Liu and Herbert, 2004;Marlow et al, 2000;McClymont and Rosell-Melé, 2005;McClymont et al, 2008;Medina-Elizalde and Lea, 2005;Sexton and Barker, 2012) it is not clear whether this was effective in driving climate changes which were conducive to the emergence of longer, higher amplitude 100 ka ice volume cycles. It is also possible that the proposed northern hemisphere ice-sheet expansion was controlled…”

“…Between 1.5 and 1.0 Ma, the observed SST cooling of glacial stages records is accompanied by cooling of the abyssal Atlantic (Figure 2) (Sosdian and Rosenthal, 2009), elevated dust input to the Subantarctic Atlantic ( Figure 5) (Diekmann and Kuhn, 2002;Martinez-Garcia et al, 2011), and more expansive subpolar water masses in the Subantarctic Atlantic followed shortly thereafter by the Pacific and north Atlantic (Martinez-Garcia et al, 2010;McClymont et al, 2008). The efficiency of the biological pump in the modern Southern…”

“…Conte et al, 1992;Sawada et al, 1998), and Pleistocene evolutionary events in coccolithophorids appear not to affect the U K 37 ' relationship to SST (McClymont et al, 2005). Due to concerns regarding the quantification of SST in the Nordic Seas in the presence of high (>5%) concentrations of the C 37:4 alkenone (Bendle and Rosell-Melé, 2004), we use the U K 37 results from Site 983 in the northern North Atlantic (McClymont et al, 2008). An alkenone SST record from site 806 is not…”

“…Evidence for unusual interglacial warmth during MIS 31 is found in the Nordic Seas (Helmke et al, 2003), northern north Atlantic (McClymont et al, 2008), the Antarctic margins (Scherer et al, 2008) and in the deep SW Pacific (Elderfield et al, 2012). Model simulations of the West Antarctic ice sheet show that a collapse during MIS 31 was particularly sensitive to the strong austral summer insolation (Pollard and DeConto, 2009) To fully test the mechanisms which are responsible for driving the premature 100 ka cycle, the cooling to 0.9 Ma, the MIS 24-22 event and the subsequent evolution of the 100 ka cycles requires renewed focus on generating spatially distributed and high resolution archives of SSTs and associated proxies for ocean circulation change.…”