Mid-Holocene deepening of the Southeast Pacific oxycline

Abstract

This study presents new high resolution sedimentary δN 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 δN 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 ka BP. In contrast, the northern Peruvian cores located on the edge of the OMZ show low δN values varying from 4 to 6‰ with amplitude of only 1‰ during the same time period. δN values decrease in all the studied cores from the last deglaciation to the early Holocene (17 to 8.5 ka BP) and reach a minimum value during the mid-Holocene. The δN variations are attributed to microbial N-loss to N, e.g. denitrification and/or anammox, and the characteristic N-enriched signal that is recorded in the underlying sediments under suboxic to anoxic conditions where denitrifiers thrive. Surprisingly, δN values from cores located within the OMZ show similar values as the more northern cores located outside the OMZ between 5 and 8.5 ka 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 δN values recorded between 8.5 and 5 ka 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 Niña-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.