Palaeoceanography of the mid-latitude North East Pacific : during the late Pleistocene

Past climatic and oceanographic changes in the California Current region off western North America were investigated with the overall goal of contributing to a better understanding of natural ocean variability in the Pacific Ocean on glacial-interglacial and millennial-time scales. This was carried out by constructing downcore sedimentary records of past primary production (C[sub org], biogenic opal, Ba/Al ratio), the concentrations of redox sensitive metals (Mn, Re, Ag, Cd, Mo, U) and the stable isotopic composition of nitrogen (¹⁴N/¹⁵N) in two sediment cores from the continental margin off Oregon (at 2730 and 3111 m water depth). These data were then compared with previously published core records to build a synoptic picture of surface, subsurface and deep water conditions in the NE Pacific over the last 120 kyrs. Isotopically enriched nitrate currently produced by denitrification in the Eastern Tropical North Pacific (ETNP) is exported northward along the continental margin via the California Undercurrent (CUC). Thus, the nitrogen isotopic composition (δ¹⁵N) of subsurface nitrate is higher than the global deep water average (-4.5-5 %o) and it decreases progressively from Mexico (18 %o) to Vancouver Island (6 %o) along the entire margin. Off Oregon, the similarity between the isotopic composition of subsurface (150 -600 m) nitrate (7.1 %o), annual sediment trap material (7.0-7.4 %o on average) and surface sediments (6.5-8%o) implies that biological nitrate uptake is complete on an annual basis and that the surface sediments record the isotopic composition of subsurface nitrate without significant diagenetic bias. Together with similar findings from other sites along the margin, these results imply that the sedimentary δ¹⁵N signal can be used as a tracer for the advection of isotopically enriched nitrate from the ETNP along the entire NW American margin from at least 2 0 ° to 45°N. Downcore δ¹⁵N results off Oregon reveal a glacial-interglacial pattern that is remarkably similar in timing and amplitude to other records from the NW American margin. High values (7-10 % o ) during the Holocene, Stage 5 and some periods of Stage 3 at the Oregon sites are interpreted to reflect relatively strong denitrification in the ETNP and a strong CUC. High sedimentary concentrations of palaeoproduction indicators imply that coastal upwelling and northerly winds off Oregon were active at the same time. This is in marked contrast to lower δ¹⁵N values (4-6 % o ) and reduced concentrations of palaeoproduction tracers during cold Stages 2 and 4. While palaeoproduction proxies and δ¹⁵N vary approximately in phase in core records originating from outside the modern denitrification zone, δ¹⁵N clearly leads palaeoproduction proxies in cores from within the ETNP after the last glacial maximum (LGM) and at the Stage 4/3 and 6/5 boundaries by several kyrs. This lead strongly implies that primary production in the ETNP cannot be the sole control on denitrification in this region. Circulation changes in the equatorial Pacific are offered as an alternative explanation. Stronger trade winds during cold periods of the climate system (such as the LGM, Heinrich events and Dansgaard-Oeschger stadials) would force a stronger equatorial undercurrent (EUC) and increase oxygen advection into the ETNP, thereby reducing denitrification rates. During warm periods, oxygen advection by the EUC is low, denitrification rates in the ETNP are high and the CUC transports isotopically heavy nitrate towards the north, thereby synchronizing the sedimentary δ¹⁵N signal along the continental margin of NW America. The proposed mechanism provides an atmospheric link between denitrification intensity in the eastern Pacific, horizontal advection of isotopically heavy nitrate into the mid latitude N-Pacific and northern hemisphere climate change. Furthermore, using sedimentary δ¹⁵N as a quasi synchronous tracer along the margin, a latitudinal gradient in the onset of primary production (as recorded by %organic carbon in the same core records) after the LGM is evident. Thus, primary production and, by inference, upwelling favourable, northerly winds appear to have developed in a time transgressive fashion with those in the north (~40°N) starting several kyrs before those in the south (~20°N). At 2730 m water depth off Oregon, the downcore authigenic enrichments of Re (up 60 times relative to crust), Ag and Cd (2-7 x) together with the absence of solid phase Mn imply that suboxic sediment conditions were maintained at shallow depth throughout the last 70 kyrs at this site. Similar to intermediate water depths, suboxic conditions at this deep site were most intense (i.e. the redox boundary was at its shallowest position) during the Holocene, a period possibly correlative with the Boiling-Allerad, and Stage 3 when organic matter flux was high. Although Mo and U in the same core are not significantly enriched over crustal reference values, their concentrations nevertheless correlate positively with production proxies, which possibly implies small amounts of authigenic Mo and TJ formation. No correlation was found between variations in the sedimentary concentrations of redox sensitive metals and the carbon isotopic composition (δ¹⁵C) of the benthic foraminifera C. wuellerstorfi, which was previously interpreted to reflect short-term changes in deep water ventilation [Mix et al., 1999; Lund and Mix 1998]. While this does not contradict the δ¹⁵C evidence that such ventilation changes might have occurred, it implies that they have not had an overriding control on the redox conditions of the sediment at this site. Overall, the Cahfornia Current region was vastly different from today under glacial climate conditions. Surface primary production and carbon export to the sediment were significantly reduced, as was the lateral advection of isotopically heavy nitrate from the ETNP by the California Undercurrent.

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