On the glacial-interglacial variability of upwelling, carbon burial and denitrification on the Northwestern Mexican continental margin

• Main Author: Ganeshram, Raja S.
• Format: Text
• Language: English
• Published: The University of British Columbia 1996
• Subjects: Pacific; Ice Sheet
• Online Access: https://dx.doi.org/10.14288/1.0087932; https://doi.library.ubc.ca/10.14288/1.0087932

Glacial-interglacial variability in upwelling on the NW Mexican margin is assessed by reconstructing the history of organic carbon and biogenic opal deposition and measuring the Ba/Al ratio in three piston cores that span the upper to the lower continental slope. Rates of accumulation of organic carbon, opal and to some degree biogenic barite are higher in interglacial intervals, indicating that upwelling-induced productivity was higher during the warm periods over the last 140,000 years. Despite cyclic changes in organic carbon accumulation, matrix-corrected HI values in the mid- and lower- slope cores are invariant and are similar to values in the laminated intervals from the oxygen-minimum site. This suggests that changes in organic carbon content are controlled by productivity variations and are not due to differential preservation induced by variations in bottom water oxygen concentrations. The lowest HI values in Mexican Margin sediments occur concurrently with large increases in grain size. Thus, increased degradation resulting from winnowing is offered as the leading explanation for the hydrocarbon impoverishments in the bioturbated upper slope deposits. Late Quaternary records of denitrification in the oxygen-deficient subsurface water masses of the Eastern Tropical North Pacific (ETNP) are constructed using ¹⁵N/¹⁴N ratios measured on bulk sediments. The profiles show a synchronous decrease in denitrification during the glacial periods over the last 140 kyrs. It is suggested that, because nitrate is a limiting nutrient in the modern ocean, a consequent increase in the oceanic nitrate inventory could have contributed to the observed decrease in glacial atmospheric pCO₂ by enhancing the fertility of the ocean. The glacial decreases in denitrification in the ETNP are attributed to large reductions in upwelling-induced fluxes of organic detritus on the margin in response to glacial shifts in the wind field off NW Mexico associated with the growth of Laurentide ice on northern North America, the establishment of a resident high pressure cell over the ice sheet, and the bifurcation of the Jet Stream.