Trace elements V, Ni, Mo and U: A geochemical tool to quantify dissolved oxygen concentration in the oxygen minimum zone of the north-eastern Pacific

Abstract

Deoxygenation of the water column in the oceans and in the oxygen minimum zone (OMZ) has become relevant due to its connection with global climate change. The variability of the OMZ has been inferred by in situ measurements for the last 70 years and qualitatively assessed through the monitoring of trace elements and the nitrogen stable isotope ratio (δ¹⁵N) of organic matter on several time scales. The V, Ni, Mo and U concentrations in surface sediments and the dissolved oxygen concentration in the water column of La Paz Bay and the Mazatlán margin were used to propose an exponential regression model. This model will allow the inference of the dissolved oxygen concentration in the sedimentary records from the Alfonso Basin in La Paz Bay and in the Mazatlán margin over the last 250 years. Based on this exponential regression model, the dissolved oxygen concentration increased by 6.4 μM in the Alfonso Basin and 4.1 μM in the Mazatlán margin, in the period between 1800 CE and the present day. This suggests a reoxygenation of the OMZ at the mouth of the Gulf of California. This finding is consistent with previous studies (Deutsch et al., 2014; Tems et al., 2016), which suggest a reduction in the deoxygenation of the water column throughout most of the 20th century.