Centennial to millennial-scale changes in oxygenation and productivity in the Eastern Tropical South Pacific during the last 25,000 years

Abstract

International audienceOxygen minimum zones (OMZ) have expanded in all tropical oceans during the last 50 years resulting in habitat contraction and considerable changes in marine biogeochemistry. However, for a better understanding of the OMZ dynamics under the current climate change, two questions are relevant: 1) how do the magnitude and temporal changes in oceanic dissolved oxygen of the last few decades compare to the natural variability on longer timescales, and 2) what were the local and remote factors driving OMZ changes in the past. In the present study we use a stacked record covering the last 25 kyr from the Eastern Tropical South Pacific (ETSP) OMZ to reconstruct changes in oxygenation and productivity. We use a suite of proxies including the presence of laminations, redox sensitive metals (U, Mo, Re, Ni and Cu), total organic carbon and δ¹⁵N measurements. Water column denitrification and sediment redox conditions show pronounced centennial to millennial-scale variability during the last 25 kyr, with oxygenation levels as low as at present. Global cold periods at different timescales such as the Last Glacial Maximum (23–19 kyr BP) and the Little Ice Age (1500–1850 AD) were associated with a weak OMZ and low export production, while warm intervals such as the deglaciation, part of the Medieval Climate Anomaly and the last 100 years are associated with a stronger OMZ and high export production. Water column denitrification and sediment redox conditions were strongly coupled during the last 25 kyr BP apart from one remarkable exception: during the Antarctic Cold Reversal, sediments were less reducing but the water column denitrification was high resulting in a strong but shallow OMZ. This may have been produced by an enhanced Antarctic Intermediate Water flow. Contrary to our expectations and modeling predictions for the next few decades, we observe a weak ETSP-OMZ during the warm mid-Holocene, which may have been the result of a stronger Walker Circulation that brought oxygen-rich waters to intermediate depths off Peru via Equatorial undercurrents. In combination with other paleoceanographic reconstructions, our results show that oxygenation variability in the ETSP-OMZ was influenced by ocean circulation changes in the Tropical Pacific, high latitude oceanographic and climatic changes, and local productivity

The simulated climate of the Last Glacial Maximum and insights into the global marine carbon cycle

“…The increase in dissolved oxygen in O-LGM was considerable but agreed well with proxy reconstructions for the upper ocean. The oxygen-poor intermediate waters of the western North Pacific (Ishizaki et al, 2009;Shibahara et al, 2007), eastern North Pacific (Cannariato and Kennett, 1999;Cartapanis et al, 2011;Chang et al, 2014;Dean, 2007;Nameroff et al, 2004;Pride et al, 1999;Ohkushi et al, 2013;van Geen et al, 2003), eastern South Pacific (Martinez et al, 2006;Muratli et al, 2010;Salvatteci et al, 2016), equatorial Pacific (Leduc et al, 2010) and Indian Ocean (Reichart et al, 1998;Suthhof et al, 2001;van der Weijden et al, 2006) were better oxygenated at the LGM relative to the PI climate. An important consequence of oxygenating the upper ocean is a reduction in the strength of denitrification in these regions.…”

“…The net decline in export production observed in this study was dominated by the decline in tropical and subtropical waters. Many palaeoproductivity studies located outside of the subantarctic zone have found weakened productivity at the LGM (Chang et al, 2014(Chang et al, , 2015Costa et al, 2016;Crusius et al, 2004;Kohfeld et al, 2005;Kohfeld and Chase, 2011;Jaccard et al, 2005;McKay et al, 2015;Ortiz et al, 2004;Riethdorf et al, 2013;Salvatteci et al, 2016;Singh et al, 2011;Thomas et al, 1995). Additionally, an enhanced utilisation of available nutrients in the subantarctic zone would reduce the nutrient content of intermediate waters formed in the Southern Ocean and would thus reduce the delivery of nutrients to lower latitudes (Sarmiento et al, 2004).…”

Deglacial to Holocene Ocean Temperatures in the Humboldt Current System as Indicated by Alkenone Paleothermometry

“…Thus, the relatively low productivity during the Mid-Holocene (Figure 3f) was probably a result of the upwelling of oxygen-rich and nutrient-poor subsurface waters. Increased subsurface oxygenation during the warm Mid-Holocene, according to the sedimentary record ( Figure 3g; Salvatteci et al, 2016) and paleoclimate models (Segschneider et al, 2018;Xu et al, 2015), is thus at odds with the expected subsurface deoxygenation in a warmer world. Increased subsurface oxygenation during the warm Mid-Holocene, according to the sedimentary record ( Figure 3g; Salvatteci et al, 2016) and paleoclimate models (Segschneider et al, 2018;Xu et al, 2015), is thus at odds with the expected subsurface deoxygenation in a warmer world.…”

“…The Br/Ti is commonly used as a semiquantitative estimate of sedimentary total organic carbon and has been successfully employed in marine sediments collected off Peru (Scholz et al, 2014). Mo is a redox sensitive metal that is used to reconstruct past changes in sediment redox conditions (e.g., Salvatteci et al, 2016).…”

“…(c) Zonal SST gradient anomaly to the Late Holocene calculated as the difference of the western and eastern Pacific averages(Koutavas & Joanides, 2012). (g) Water column denitrification inferred from δ 15 N on sedimentary organic matter, data taken fromSalvatteci et al (2016), and the ratio of molybdenum to titanium in core M135-004-3 as a proxy for sediment redox conditions (this study). (e) Mean annual SST from fossil mollusk δ 18 O values on the Peru coast(Carré et al, 2014).…”