Group items tagged

Quantitatively assessing the porewater dissolved inorganic carbon (DIC) cycling in methane-enriched marine sediments is crucial to understanding the contributions of different carbon sources to the global marine carbon pool. In this study, Makran accretionary wedge was divided into Zone 1 (high methane flux area) and Zone 2 (background area). Porewater geochemical compositions (Cl-, SO42-, NH4+, Mg2+, Ca2+, Ba2+, DIC and δ13C-DIC) and a reaction-transport model were used to determine the DIC source and calculate the DIC flux through carbonate precipitation and releasing into overlying seawater in sediments. Zone 1 is characterized by the shallower depth of sulfate-methane transition (SMT), where most of porewater sulfate was consumed by anaerobic oxidation of methane (AOM). In contrast, a relatively low flux of methane diffusion in Zone 2 results in a deeper SMT depth and shallow sulfate is predominantly consumed by organoclastic sulfate reduction (OSR). Based on the porewater geochemical profiles and δ13C mass balance, the proportions of porewater DIC originating from methane were calculated as 51% in Zone 1 and nearly 0% in Zone 2. An increase of porewater DIC concentration leads to authigenic carbonate precipitation. Solid total inorganic carbon (TIC), X-ray diffractometry (XRD) and scanning electron microscopy (SEM) analysis display that carbonate content increases with depth and aragonite appears at or below the depths of SMT. Meanwhile, the flux of DIC released from sediments calculated by the reaction-transport model is 51.3 ~ 90.4 mmol/m2·yr in Zone 1, which is significantly higher than that in Zone 2 (22.4 mmol/m2·yr). This study demonstrates that AOM serves as the dominant biogeochemical process regulating the porewater DIC cycle, which has an important impact on the authigenic carbonate burial and the seawater carbonate chemistry.

The Southern Indian Ocean is a major reservoir for rapid carbon exchange with the atmosphere, plays a key role in the world's carbon cycle. To understand the importance of anthropogenic CO2 uptake in the Southern Indian Ocean, a variety of methods have been used to quantify the magnitude of the CO2 flux between air and sea. The basic approach is based on the bulk formula-the air-sea CO2 flux is commonly calculated by the difference in the CO2 partial pressure between the ocean and the atmosphere, the gas transfer velocity, the surface wind speed, and the CO2 solubility in seawater. However, relying solely on wind speed to measure the gas transfer velocity at the sea surface increases the uncertainty of CO2 flux estimation. Recent studies have shown that the generation and breaking of ocean waves also significantly affect the gas transfer process at the air-sea interface. In this study, we highlight the impact of windseas on the process of air-sea CO2 exchange and address its important role in CO2 uptake in the Southern Indian Ocean. We run the WAVEWATCH III model to simulate surface waves in this region over the period from January 1st 2002 to December 31st 2021. Then, we use the spectral partitioning method to isolate windseas and swells from total wave fields. Finally, we calculate the CO2 flux based on the new semiempirical equation for gas transfer velocity considering only windseas. We found that after considering windseas' impact, the seasonal mean zonal flux (mmol/m2·d) increased approximately 10%-20% compared with that calculated solely on wind speed in all seasons. Evolution of air-sea net carbon flux (PgC) increased around 5.87%-32.12% in the latest 5 years with the most significant seasonal improvement appeared in summer. Long-term trend analysis also indicated that the CO2 absorption capacity of the whole Southern Indian Ocean gradually increased during the past 20 years. These findings extend the understanding of the roles of the Southern Indian Ocea

Shifts in phytoplankton phenology were observed in the Strait of Malacca (SM) and Sunda Shelf (SS), which were speculated to be potentially related to global warming and climate anomaly events. Such interactions between phytoplankton structure and physico-chemical factors were less known in narrow straits. Therefore, the spatial distribution pattern and diversity of surface phytoplankton assemblage, local hydrology, and nutrient regimes were investigated over the SM and SS (South China Sea, SCS) during 2017 and 2018 pre-monsoon season (spring). Diatoms, dinoflagellates, and cyanobacteria were representatives of microphytoplankton in the survey area. Total phytoplankton abundance peaked near Singapore Strait (SGS) and diminished toward SS. From the lower ratio of diatoms to dinoflagellates (<3) in SS, we deduced lower carbon pump efficiency here. In agreement with the modeled results proposed previously, cold conditions (negative Indian Ocean Dipole, IOD) were more suitable for high diatom (especially centric forms) abundance, while warm scenarios (positive IOD/El Niño period as in 2017) seemed to favor dinoflagellates and/or cyanobacteria. Specifically, diatom proportion increased by 30% and dinoflagellate, cyanobacteria reduced by 71%, 75% in response to shifts of climate anomaly from 2017 cruise to 2018 cruise. This study between field microalgae and physical and chemical conditions would be helpful to launch large-scale climate model, biogeochemistry, and carbon cycling in future research.