Time-transgressive North Atlantic productivity changes upon Northern Hemisphere glaciation

Abstract

Marine biological export productivity declined in high‐latitude regions in the North Pacific and Southern Ocean 2.7 million years ago, in parallel with the intensification of Northern Hemisphere glaciation. Here we present data from the North Atlantic, which show a similar but time‐transgressive pattern of high‐latitude productivity decline from 3.3 to 2.5 Ma, with productivity decreasing first at 69°N, hundreds of thousands of years before it declined at 58°N. We propose that the cumulative data are best explained by an equatorward migration of the westerly winds, which caused a southward shift in the zone of Ekman divergence and upwelling‐associated major nutrient supply over this time interval. We suggest that a similar equatorward migration of the westerly winds may also help explain the productivity changes observed in other high‐latitude regions, particularly the Southern Ocean. At 2.7 Ma, equatorial and temperate Atlantic sites began to show orbitally paced productivity pulses, consistent with a shoaling and meridional contraction of the nutrient‐poor “warm sphere” that characterizes the low latitude upper ocean. This timing coincides with observed productivity changes in Southern Ocean, consistent with previous findings that the Southern Ocean exerts a strong influence on the fertility of the low‐latitude Atlantic. Finally, we propose that the unique basin geometry of the North Atlantic caused deep water formation in this region to remain relatively stable despite equatorward migration of winds and ocean fronts.

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

“…During this period of time, surface waters in the study area experienced a major cooling (summarized in Lawrence et al, 2010) that was accompanied by frequent inputs of ice rafted debris (McIntyre et al, 1999). The cooling occurred during what appeared to be a time of relatively stable polar climate and glacial ice volumes (Lisiecki and Raymo, 2005; Fig. 3).…”

“…Only after about 2.7 Ma (glacial MIS G6/G4) until 2.0/1.8 Ma, the offset between the two ε Nd records at sites 980/981 and 610 significantly increased to reach on average 1.5 to 2 ε Nd units difference during a time interval when the stacked benthic to oscillate for the first time above the present interglacial level (Lisiecki and Raymo, 2005; Fig. 3).…”

“…Further work, including producing higher resolution ε Nd records and particularly more reliable pCO 2 estimates, are particularly needed to investigate whether these major deep Atlantic circulation changes may have altered the global ocean biological pump and thus the carbon storage after 1.6 Ma (e.g., Wang et al, 2010), at a point in time clearly preceding the main shift in amplitude and frequency of benthic δ 18 O (ice volume) record (Lisiecki and Raymo, 2005) Berger and Jansen, 1994).…”

“…Horizontal dashed lines mark inferred modern bottom-water ε Nd signature at study sites and the northeast Atlantic from proximally water column measurements Jeandel, 2004a, b, 2005;Rickli et al, 2009) and near-core top leachates and foraminifera (Crocket et al, 2011;Piotrowski et al, 2013; this study). The numbers on the global benthic foraminiferal δ 18 O stack LR04 of Lisiecki and Raymo (2005) correspond to glacial marine isotope stages (MIS). NADW -North Atlantic deep water, SOW -Southern Ocean water, WTROW -Wyville Thomson Ridge overflow water, and NSOW -Nordic Seas overflow water.…”

“…The results demonstrate that the authigenic ferromanganese coatings of both mixed planktonic foraminifera and of bulk sediment preserved a reliable bottom-water ε Nd signature at the investigated sites in the North Atlantic. The age model for site 610 has been adopted from De Schepper and Head (2008) and for site 980/981 from Lisiecki and Raymo (2005). The age model for site 900 integrates evidence from shipboard biostratigraphy and magnetostratigraphy of ODP Leg 173 (Zhao et al, 2001).…”