Laminated sediments from the central Peruvian continental slope: A 500 year record of upwelling system productivity, terrestrial runoff and redox conditions

Sedimentological studies including X-ray digital analyses, mineralogy, inorganic contents, and organic geochemistry on cores of laminated sediments accumulated in the oxygen minimum zone of the central Peruvian margin reveal variable oceanographic and climate conditions during the last 500 yr. Coherent upcore variations in sedimentological and geochemical markers in box cores taken off Pisco (B0405-6) and Callao (B0405-13) indicate that variability in the climate proxies examined has regional significance. Most noteworthy is a large shift in proxies at ∼1820 AD, as determined by ²¹⁰Pb and ¹⁴C radiometric dating. This shift is characterized by an increase in total organic carbon (TOC) in parallel with an abrupt increase in the enrichment factor for molybdenum Mo indicating a regional intensification of redox conditions, at least at the sediment water interface. In addition there was lower terrestrial input of quartz, feldspar and clays to the margin. Based on these results, we interpret that during several centuries prior to 1820, which corresponds to the little ice age (LIA), the northern Humboldt current region was less productive and experienced higher terrestrial input related to more humid conditions on the continent. These conditions were probably caused by a southward displacement of the inter-tropical convergence zone and the subtropical high pressure cell during the LIA. Since 1870, increases in TOC and terrigenous mineral fluxes suggest an increase of wind-driven upwelling and higher productivity. These conditions continued to intensify during the late 20th century, as shown by instrumental records of wind forcing.