Nutrient leakage from the North Pacific to the Bering Sea (IODP Site U1341) following the onset of Northern Hemispheric Glaciation?

Abstract

Intergrated Ocean Drilling Program Expedition 323 recovered a sediment record covering the last ~4.3 Ma from the Bering Sea (Integrated Ocean Drilling Program Site U1341, Bowers Ridge, 2177 m water depth). To resolve Pliocene‐Pleistocene paleoenvironmental changes in this marginal basin, ~190 sediment samples were analyzed for their bulk element composition. Aluminium contents in Bowers Ridge sediments are variable but overall higher toward younger sediments, probably related to the intensification of the Northern Hemispheric Glaciation and increasing sea ice transport in the Bering Sea. The gradual increase of terrigenous input is mirrored by decreasing SiO2 and excess Si (Sixs) contents, but the overall Si enrichment of the deposits reflects continuous opal deposition since the Pliocene at Bowers Ridge. Unlike in the North Pacific, the Sixs record at Site U1341 does not support a dramatic decrease in opal export following the onset of the Northern Hemispheric Glaciation around 2.7 Ma, but SiO2xs have higher accumulation rates (up to ~8 g/cm2/ka) between ~2.6 and ~1.8 Ma BP. During this period, the major oceanic opal deposition centers shifted globally from open marine high latitude regions to upwelling areas. We here discuss how the onset of North Pacific stratification at ~2.7 Ma BP may have caused leakage of nutrient‐rich deep/intermediate North Pacific water into the Bering Sea via the deep Kamtchatka Strait, leading to increased opal deposition—and likely reactive organic matter export—at Bowers Ridge. As a result, magnetic and geochemical records were overprinted by intensified diagenesis, significantly affecting their applications as paleoceanographic proxies.

Equatorial Pacific peak in biological production regulated by nutrient and upwelling during the late Pliocene/early Pleistocene cooling

“…This indicates that the EF sat well to the north of its present position, and the two sites were overlain by the nitrate-rich cold tongue, which is consistent with foraminifera data reconstructions (Steph et al, 2009). The intensification of the EEP upwelling is probably linked to the shoaling of the thermocline, the development of a strong zonal and meridional SST gradient in tropical/subtropical regions, which may have enhanced atmospheric circulation and divergent upwelling along the Equator, and brought the nutrient-rich polar water masses escaping the poorly used nutrient pool of the Southern Ocean and North Pacific within reach of the upwelling circulation (Billups et al, 2013;Etourneau et al, 2010;März et al, 2013).…”

“…In addition, the paleoposition of the two ODP Sites 1239 and 1240 has changed negligibly over the last ∼ 4.0 Ma with respect to oceanographic position (Gripp and Gordon, 1990;Pisias et al, 1995;Mix et al, 2003). Therefore, our records at both EEP sites must reflect the regional export production variability related to changes in regional oceanic conditions and nutrient supply during the Plio-Pleistocene climate transition.…”

“…Cacho, personal communication, 2012). Paleomagnetic and biostratigraphic data were used to constrain the intermediate ages (Mix et al, 2003).…”

“…However, although the age model at our EEP sites is well constrained, the use of MAR instead of content can be largely uncertain regarding the export productivity produced in the subsurface waters (François et al, 2004). In addition, the carbonate (CaCO 3 ) content has not varied significantly since 3.0 Myr (Mix et al, 2003), implying that changes in dilution or concentration of the other more minor biogenic components by carbonate fluctuations are not driving the fluctuations of the records. The exception may be during the high TOC/TN/C 37 interval, where CaCO 3 content decreases slightly (Mix et al, 2003).…”

“…In addition, the carbonate (CaCO 3 ) content has not varied significantly since 3.0 Myr (Mix et al, 2003), implying that changes in dilution or concentration of the other more minor biogenic components by carbonate fluctuations are not driving the fluctuations of the records. The exception may be during the high TOC/TN/C 37 interval, where CaCO 3 content decreases slightly (Mix et al, 2003). This probably represents a decrease in CaCO 3 preservation due to high TOC delivery, but it was relatively minor compared to the high productivity-related concentrations peak centered between 2.2 and 1.6 Myr.…”