Interplay between sedimentary organic matter and dissolved oxygen availability in a coastal zone of the Humboldt Current System; Mejillones Bay, northern Chile

Geochemical and petrographical analyses were done in 26 surficial sediment samples from Mejillones Bay (northern Chile) to investigate the processes that participate in the preservation of sedimentary organic matter. Annual variability of chlorophyll-a and DIC δ13C measured in surface waters of three different parts of Mejillones Bay did not show any significant spatial variability in biological productivity. On the other hand, bottom dissolved oxygen showed strong changes associated with water depth and season. Surface sediment composition was statistically analyzed displaying organic facies, which are in agreement with the principal zones of bottom oxygenation (annual mean condition) identified in this bay. In the oxic coastal zone (> 1 ml l− 1) the total carbon (TC), total nitrogen (TN), total organic carbon (TOC), hydrogen index (HI) and oxygen index (OI) were 1.3%, 0.1%, 0.4%, 355.8 mg HC/g TOC and 237.6 mg CO2/g TOC, respectively. These results and the presence of almost exclusively dispersed amorphous organic matter suggest an environment where most metabolizable organic matter is degraded within the water-column and/or in the sediment water interface. In the microxic hemipelagic zone (< 0.1 ml l− 1), located deeper than 65 m, the TC, TN, TOC, HI and OI were 10.5%, 1.0%, 6.6%, 529.7 mg HC/g TOC and 103.1 mg CO2/g TOC, respectively. These values and the presence of abundant agglomerated amorphous organic matter suggest a better preservation of metabolizable and non-metabolizable compounds. We can conclude that biological productivity and dissolved oxygen exert a similar influence on the sedimentary organic matter, but the characteristics and quantity of organic material preserved in bottom sediments of this bay are controlled, principally, by the availability of dissolved oxygen at the sediment–water interface. According to this conclusion, we suggest that the sedimentary record of Mejillones Bay must be interpreted in paleoceanographic terms primarily in relation to the temporal variability of dissolved oxygen, more than biological productivity. The OI seems to be a good proxy of paleoxygenation in this coastal system.