Millennial-scale variability in denitrification and phosphorus burial in the Eastern Tropical North Pacific

Abstract

The remarkable synchrony between changes in temperature recorded in Greenland ice cores and variations in N isotope records from sedimentary cores recovered from the Arabian Sea and the Eastern Tropical North Pacific (ETNP) has provided evidence for teleconnections between changes in marine denitrification in the tropics and climate variations in the northern high latitudes. Changes in tropical denitrification have been attributed to changes in productivity, changes in the source of intermediate waters and the flux of dissolved oxygen to suboxic zones. Variations in marine denitrification and anammox occurring at intermediate depths in proximity to productive continental margins have had profound effects on the N:P ratio of upwelled waters between stadials and interstadials, and may have indirectly affected carbon sequestration in the ocean by changing the balance of nutrients available to primary productivity. Competitive equilibrium, the changing stoichiometric balance of elements available as nutrients and the shorter residence time of N compared to P are factors that are believed to favour diazotrophs (N2-fixing organisms) during interstadials and shift the competitive advantage to non-N2-fixing ecosystems during stadials. This study presents a very high-resolution analysis of sedimentary nitrogen isotope records, phosphorus concentrations and bulk detrital element concentrations from two cores collected along the Pacific Mexican Margin. The results show that the oxygen minimum zone (OMZ) bathing intermediate waters in ETNP is modulated by the interaction of a Northern Hemisphere climate component with the “leakage” of heavy nitrate believed to derive from the Eastern South Pacific (ESP). This southerly component has a more “Antarctic” timing and is similar to records from the Peru-Chile margin. The sedimentary core recovered from the Mazatlan margin shows a “Greenland” timing of millennial-scale events, with reduced upwelling and reduced primary productivity, a less intense OMZ leading to reduced denitrification and a more southerly position of the mid-tropospheric subtropical ridge during stadials. This would have increased the onshore flow of moist air, ultimately leading to increased precipitation along the western Mexican Margin. Interstadials show a reversal of these conditions. In contrast to the Mazatlan core, the N isotope record from the core recovered from the Gulf of Tehuantepec records an element of “Antarctic” timing superimposed on local, millennial-scale variations in denitrification that are more similar in timing to Greenland temperature changes. In addition, the interpretation of observed variations in detrital elements from the Gulf of Tehuantepec highlights latitudinal displacements of the ITCZ that are consistent with those observed in the Cariaco Basin in Venezuela. Bulk P concentrations from both cores suggest that although phosphorite formation in the ETNP during interstadials is not as widespread as previously thought, the very high accumulation rates in the Gulf of Tehuantepec and Mazatlan Margin lead to total Holocene phosphorus burial rates that are up to 4-5 times higher than had been estimated in previous studies. These observations lead to the argument that the ETNP may play a more important role in regulating global P budgets than was previously thought and call for an improved appreciation of the benthic microbial communities that modulate biomes at tropical latitudes.