Peruvian Oxygen Minimum Zone dynamics during the last 18 000 years
The Eastern Tropical South Pacific (ETSP) subsurface waters are affected by one of the strongest and shallowest oxygen minimum zone (OMZ) in the World Ocean, which results from the interplay of high regional primary production (PP) and sluggish subsurface ventilation. The recent expansion of the tropical OMZs suggests that these zones are very vulnerable to anthropogenic climate change, but their sensitivity to changes in PP and in nutrient cycling still remains poorly understood and under debate. We present a core-top compilation of nitrogen isotopes (d15N) measured on a collection of surface sediment retrieved along the Ecuadorian and Peruvian coasts and compare it to the upper 200 m of water column samples to identify which processes may ultimately influence downcore (i.e past) changes in the isotopic signature of bulk nitrogen. We overall find that in O2–depleted subsurface waters, microbial-mediated NO3- loss to N2 leaves a 15N-enriched signal in underlying sediments characteristic of water masses affected by denitrification and/or anammox, so called nitrogen-loss (N-loss) processes. We find that phytoplankton nitrate (NO3-) uptake in surface waters contributes significantly to the sedimentary signal within the high nutrient, low chlorophyll (HNLC) areas of the studied region. We also detect regional differences in the water column and sedimentary signatures of nitrogen isotopes with respect to contrasting oxygenation and PP status. In the North, between 1°N-10°S, subsurface O2 concentrations are too high to allow N-loss processes to take place and account for nitrogen isotopic changes, keeping !15NNO3- values relatively low in the subsurface waters. In contrast, d15NNO3- values increase toward the surface due to partial nitrate utilization in the photic zone. However, an incomplete utilization of NO3- probably triggers d15Nsed values being consistently lower than the !15N of the upwelled NO3-. Further south, between 10 - 15°S within the perennial upwelling region, HNLC conditions are relaxed, resulting in more intense biological production and its associated phytoplankton uptake of surface NO3-. In addition, subsurface O2 concentration decreases to levels sufficient for N-loss by denitrification and/or anammox to take place, resulting in elevated subsurface d15NNO3- values in waters feeding the coastal upwelling. Various sedimentary proxies for productivity reflect increasingly higher PP southward, and echoe a parallel north-south d15Nsed gradient reflecting an increasing subsurface N-loss southwards. In addition, the d15N values measured on core-top sediments and on water column samples increase cross-shelf toward the coast, and highlight an intensification of coastal upwelling along the shoreline bringing the oxycline and nitracline closer to the surface. Overall, our mapping effort suggests that the degree of N-loss influencing subsurface d15NNO3- values, the degree of upper ocean stratification, and the degree of near-surface nitrate utilization should all be considered for the interpretation of downcore d15Nsed records in an attempt of reconstructing the spatio-temporal variability of the Peruvian OMZ. Following on this, past changes in the extent of the OMZ were reconstructed on the basis of 4 piston cores selected from regions located outside, at the rim and within the core of the modern OMZ along a latitudinal transect (from 3.5 to 15°S). The !15N values measured on each core cover the entire Holocene and the last deglaciation period. In the northern part along the Ecuadorian margin, the d15N values are low, ranging from 4 to 6‰, with an amplitude of ~1‰ during the Holocene and the last deglaciation period. This is likely due to the absence of OMZ conditions within this area. In contrast, the southern cores show values similar to modern core-tops data and a large amplitude (~4‰) over the past, that we interpret as resulting from variable N-loss processes taking place in the core of the OMZ, from modern time up to the beginning of the last deglaciation. In all the studied cores, we generally observe a strong increase first and then a decreasing trend in the d15N values a along the time period of the deglaciation, followed by relatively stable d15N signatures during the mid-Holocene. Interestingly, low !15N values (~5.5‰) consistently characterize all the studied cores for the mid-Holocene time interval, indicating an overall absence of heavy nitrate in the surface being subsequently uptaken by phytoplankton. As the proxies for export production suggest that there is no straightforward correlation with d15N values during the mid-Holocene, the changes in d15N values are more likely resulting from modifications of subsurface ventilation rather than of nutrient utilization. An increase in upwelling intensity during the mid-Holocene may have allowed for a more efficient ventilation of the upper part of the water column,preventing any N-loss processes to take place, along with injection of NO3- with low d15N within the euphotic zone. A mid-Holocene increase in upwelling intensity, analogously to modern conditions prevailing during austral winters and La Niña events, should also be mirrored in sedimentary records sensitive to regional shifts in the atmospheric circulation such as latitudinal shifts in the Intertropical Convergence Zone (ITCZ). We further investigated how past changes in precipitation along the Ecuadorian margin has evolved over the last 18 000 years. We used two high-resolution XRF records to track downcore changes in the Guayas River runoff. The titanium over calcium ratio is supposed to reveal information on paleo-precipitation shifts along the western flanks of the equatorial Andes. When compared to other deglacial to Holocene rainfall records located across the tropical South American continent, different modes of variability become apparent. The records of rainfall variability imply that changes in the hydrological cycle at orbital and sub-orbital timescales were different from western to eastern South America. Orbital forcing caused an antiphase behavior in rainfall trends between eastern and western equatorial South America. In contrast, millennial-scale rainfall changes, remotely connected to the North Atlantic climate variability, led to homogenously wetter conditions over eastern and western equatorial South America during North Atlantic cold spells. These results may provide helpful diagnostics for testing the regional rainfall sensitivity in climate models and help to refine rainfall projections in South America for the next century, and corroborate the mid-Holocene increase in upwelling activity which was likely responsible for pushing the ITCZ north of the Ecuadorian margin at this time.
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