Venti & Billups (2013), Surface water hydrography of the Kuroshio Extension during the Pliocene-Pleistocene climate transition

Despite its vast size and key role in climate, relatively little is known about the Pacific’s paleoceanographic evolution. Since the late Neogene, the Pacific sea floor has lain largely beneath the carbonate compensation depth such that continuous carbonate sections from this interval are rare beyond the equatorial region. This sedimentation pattern stymies many traditional dating approaches; planktic foraminifer and calcareous nannofossil biostratigraphy and benthic foraminifer oxygen isotope stratigraphy all require good carbonate preservation.

 

However, at Shatsky Rise an extraordinary volume of effusive volcanic material provides a seafloor environment sufficiently broad and shallow for accumulation of thick and continuous carbonate-rich sediment from the Late Neogene onward, thereby allowing excellent age control. Serendipitously, Shatsky Rise underlies the Kuroshio Current Extension (KCE). This is the North Pacific’s primary oceanographic feature, a powerful western boundary current that insulates the warm subtropical gyre from cooler waters in the mid-latitude and subarctic regions. In the modern ocean, transfer of heat from the warm KCE to the cold atmosphere during winter (>100Wm-2) is sufficient to cool the sea surface by 10°C and virtually erases the thermocline. Only ocean-atmosphere heat transport from the Gulf Stream exceeds that in the NW Pacific. Thus Shatsky Rise provides an ideal location to investigate the Neogene evolution of the North Pacific in a climatic context.

 

Here we examine the KCE’s relationship to the Plio-Pleistocene onset of Northern Hemisphere glaciation (NHG). To reconstruct KCE hydrography (temperature and salinity), we measured δ18O in a planktic foraminifer, Globigerinoides (Gs.) ruber, at high resolution (2500-year time step) at Ocean Drilling Program Site 1208 (36.1°N, 158.5°E; panel b). This is to date the longest Plio-Pleistocene oxygen isotope record from a planktic foraminifer at such high resolution in the open North Pacific. As Gs. ruber requires warm temperatures, i.e., the KCE’s warm mixed layer during summer and fall, this measurement should largely reflect heat transported to the mid latitudes in the subtropical gyre via the powerful current. To more clearly resolve surface hydrography (Δδ18O; panel c), we remove secular δ18O variability of the marine reservoir using an approximation based on our previously published companion benthic δ18O record (panel d). The summer/fall hydrographic reconstruction reveals increased temperatures and thus suggests increased heat availability in the Kuroshio Extension after NHG onset at 2.7 Ma. Though this may seem counterintuitive, these warmer KCE waters would have potentially provided an important moisture source for snow fall in North America. On the orbital timescale, KCE hydrography (likely temperature) varied with overhead insolation during summer/fall via precession (panel a), as is common in subtropical records. In contrast, high-latitude climate, as represented by the benthic δ18O record, followed obliquity, illustrating the insensitivity of the Kuroshio Current system to climate change at higher latitudes.

 

Click here to go to the SedDB data record.

 

 

 

Comparison of Site 1208 Globigerinoides ruber δ18O measurements (b) to summer (June 21–July 20) insolation at 36°N (a; Laskar et al., 1993), Δδ18O, a measure of sea surface hydrography (c, see text for details), and the Site 1208 benthic δ18O record (d). The gray curve in (d) reflects the 10-kyr running mean (see text for details). Individual Gs. ruber δ18O measurements plot as dots (b); the line connects sample averages. The shaded interval and arrow in (d) indicates the onset of NHG. Gray lines in (c) show hydrographic means before and after NHG onset.

 

Surface water hydrography of the Kuroshio Extension during the Pliocene-Pleistocene climate transition, Nicholas L. Venti and Katharina Billups, Marine Micropaleontology, 2013; doi:10.1016/j.marmicro.2013.02.004
 

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