Séminaire : Dimitri Guttiérez (Instituto del Mar del Peru (Callao, Peru) et Universidad Peruana Cayetano Heredia (Lima, Peru)): Recent trends of subsurface ocean oxygenation in the Eastern Tropical South Pacific: insights from proxy records and instrumental observations
Monday 17 June 2019, 11:00am - 12:00pm
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Abstract : The Eastern Tropical South Pacific (ETSP) oxygen minimum zone (OMZ) is one of the largest regional OMZs in the global ocean. At multidecadal to millennial scales, the intensity and extension of this OMZ is modulated by changes of the Walker circulation, which control the thermocline depth–and thus productivity and respiration–in the ETSP, as well as the subsurface ventilation associated to the equatorial currents. Further south, the OMZ is also influenced by the meridional shifts of the subtropical front and by the intensity of the South Pacific Subtropical High (SPSH), which control the ventilation at higher latitudes and the alongshore wind forcing, respectively. For the late Holocene, proxy records of oxygenation and productivity have evidenced significant shifts at multi-centennial time-scales, associated to global ‘cold’ and ‘warm’ climatic periods. During the Little Ice Age (LIA, 1400-1850 AD), proxy records off Peru and Northern Chile indicate an increased oxygenation in the water column and lower export production, under both weak Walker circulation and SPSH conditions. After the LIA, biogenic (e.g. benthic foraminifera) and geochemical (organic matter δ15N and redox sensitive metals accumulated in the sediments) records indicate an increase in subsurface deoxygenation and a higher export production. For the twentieth century, a slight oxygenation is noticeable in the OMZ that seems decoupled from increased export production. Proxy records are supported by historic shipboard oxygen measurements since the 1960s to the 2000s, which indicate a slight trend of ventilation close to the OMZ core over the upper continental slope. These observations contrast with those from the upper water column that exhibit strong regional interdecadal variability for the same period and a shoaling trend of the oxycline for the past few decades. They also differ from some records for the North Eastern Pacific, whereby a deoxygenation trend is noticeable since 1990. We discuss some mechanisms to be examined in order to explain the observed trends as, for instance, advection of subsurface equatorial waters that could ventilate the ETSP, changes in local consumption, or northward advection of the oxygen-rich subsurface and intermediate waters from the south, (e.g. Chile-Peru Deep Coastal Current, CPDCC).
Location salle de réunion LOCEAN, tour 45/55, 4eme étage