Mid-Holocene deepening of the Southeast Pacific oxycline

Abstract

This study presents new high resolution sedimentary δ15N records from piston cores collected within and outside the present-day eastern south Pacific oxygen minimum zone along a latitudinal transect from 3.5°S to 15°S. Radiocarbon dating of foraminifera and organic matter show that the cores cover the Holocene and the last deglaciation with high sedimentation rate allowing interpretations at millennial to centennial timescale. High δ15N values, reaching 10‰ and large amplitude changes, with a magnitude of ~4‰, are observed in the southern part of the studied area during the last 18\u202fka BP. In contrast, the northern Peruvian cores located on the edge of the OMZ show low δ15N values varying from 4 to 6‰ with amplitude of only 1‰, during the same time period. δ15N values decrease in all the studied cores from the last deglaciation to the early Holocene (17 to 8.5\u202fka BP) and reach a minimum value during the mid-Holocene. The δ15N variations are attributed to microbial N-loss to N2, e.g. denitrification and/or anammox, and the characteristic 15N-enriched signal that is recorded in the underlying sediments under suboxic to anoxic conditions where denitrifiers thrive. Surprisingly, δ15N values from cores located within the OMZ show similar values as the more northern cores located outside the OMZ between 5 and 8.5\u202fka BP. This minimum is not related to local changes in export production, reconstructed from sedimentary organic carbon, total nitrogen and bromine, but appears to be controlled by changes in the ventilation of the area. The low δ15N values recorded between 8.5 and 5\u202fka BP are well correlated with more arid conditions developed along the Peruvian margin and an increase of the sea surface temperature gradient along the Peruvian margin and between the West and East Pacific along the equator, implying an intensification of the Hadley circulation and climatic conditions similar to La Nina-like state. Consequently, these mid-Holocene conditions led to greater ventilation of subsurface waters that deepened the Peruvian oxycline then revealing similar conditions as observed today in the northern part of the study area.