Efficiency Of Biological Carbon Pump Research Articles

Collapse and recovery of the marine biological carbon pump across the Cretaceous-Paleogene mass extinction

<p indent="0mm">The Cretaceous-Paleogene (K-Pg) mass extinction that ended the age of the dinosaurs occurred 66 million years ago, with competing hypotheses including the Deccan Traps volcanic eruption, climate change, and sea level change. However, it is the Chicxulub asteroid impact that has been widely accepted as the most plausible cause. Whatever the reason, the Earth’s ecological environment changed dramatically, leading to a mass extinction of both terrestrial and marine life. Primary producers lie at the base of the food chain and support higher trophic levels, so play an important role in the marine ecosystem. The predominant primary producers in the ocean, coccolithophores (phytoplankton with small CaCO₃ scales covering their cell surface), were among those most severely devastated by the K-Pg mass extinction. So, studying fossil coccolithophores across the K-Pg boundary can provide a better understanding of the destruction and post-event recovery processes of marine primary production (PP) and export production (EP), and the changes and controlling mechanisms of the marine biological carbon pump in the current global climate change. The four hypotheses previously proposed to interpret the post-event primary production, namely strangelove ocean (collapsed PP), living ocean (collapsed EP), resilient ocean (continued PP and EP), and heterogeneous ocean (geography-dependent responses of PP and EP), are illustrated and compared, and their pros and cons reviewed. A dataset is compiled from multiple locations in the global ocean basin including Deep Sea Drilling Program Site 465, Ocean Drilling Program Sites 690, 1210, and 1262, and the outcrop at El Kef (Tunisia), with each location having a high-precision cyclostratigraphic time control and an ocean surface-to-deep (bulk carbonate-benthic foraminifera) <italic>δ</italic>¹³C vertical gradient. This global dataset is analyzed to explore the changes in the efficiency of global marine biological carbon pump after the K-Pg boundary event. Three important patterns are observed: (1) Prior to the K-Pg event, the vertical <italic>δ</italic>¹³C gradient in all ocean basins falls between 0.8‰–1.5‰ and is consistent with that in the modern ocean, and this gradient seems to gradually increase from the shelf to the deep ocean possibly due to higher efficiency of the biological carbon pump in the open ocean commonly characterized by oligotrophy; (2) the global ocean vertical <italic>δ</italic>¹³C gradient collapses after the K-Pg event, suggesting drastic changes in primary and/or export production in the euphotic zone; (3) the vertical <italic>δ</italic>¹³C gradient collapse results from the abrupt negative shift of the surface ocean <italic>δ</italic>¹³C, and the magnitude of this negative shift gradually decreases from the continental shelf to the deep sea, which may be related to the selective extinction of shallow-water grazers, swimming organisms and plankton, as well as the limited buffering capacity of shelf environments. After the K-Pg event, the main primary producers change from larger coccolithophores to smaller picoplankton, and the more recalcitrant dissolved organic carbon is able to sink to the seafloor due to the operation of the microbial carbon pump in the photic zone, leading to sustained efficiency of the biological carbon pump. Although the picoplankton-dominated biological carbon pump is not as efficient as that by coccolithophores, it is still capable of removing enough nutrients and harmful substances from the photic zone via long-term operation, not only providing food for higher trophic level organisms, but improving the marine physicochemical environment that deteriorated after the K-Pg asteroid impact. This process paves the way for the restoration and development of the entire marine ecosystem. The switch of key marine primary producers and the controlling factors involved are critical scientific issues that deserve more focus in future studies of the deep-time marine carbon cycle. The regional or global variation patterns in the biological carbon pump after the K-Pg event reveal how different marine ecosystems operate under extreme environmental stress, and provide a reference for better understanding and predicting how they respond to the ongoing rapid global climate change.

Biological responses of the marine diatom Chaetoceros socialis to changing environmental conditions: A laboratory experiment.

Diatoms constitute a major group of phytoplankton, accounting for ~20% of the world’s primary production. It has been shown that iron (Fe) can be the limiting factor for phytoplankton growth, in particular, in the HNLC (High Nutrient Low Chlorophyll) regions. Iron plays thus an essential role in governing the marine primary productivity and the efficiency of biological carbon pump. Oceanic systems are undergoing continuous modifications at varying rates and magnitudes as a result of changing climate. The objective of our research is to evaluate how changing environmental conditions (dust deposition, ocean warming and acidification) can affect marine Fe biogeochemistry and diatom growth. Laboratory culture experiments using a marine diatom Chaetoceros socialis were conducted at two temperatures (13°C and 18°C) and under two pCO2 (carbon dioxide partial pressure) (400 μatm and 800 μatm) conditions. The present study clearly highlights the effect of ocean acidification on enhancing the release of Fe upon dust deposition. Our results also confirm that being a potential source of Fe, dust provides in addition a readily utilizable source of macronutrients such as dissolved phosphate (PO4) and silicate (DSi). However, elevated atmospheric CO2 concentrations may also have an adverse impact on diatom growth, causing a decrease in cell size and possible further changes in phytoplankton composition. Meanwhile, ocean warming may lead to the reduction of diatom production and cell size, inducing poleward shifts in the biogeographic distribution of diatoms. The changing climate has thus a significant implication for ocean phytoplankton growth, cell size and primary productivity, phytoplankton distribution and community composition, and carbon (C), nitrogen (N), phosphorus (P), silicon (Si) and Fe biogeochemical cycles in various ways.

Spatial variability of upper ocean POC export in the Bay of Bengal and the Indian Ocean determined using particle‐reactive 234Th

AbstractThe northern Indian Ocean is globally significant for its seasonally reversing winds, upwelled nutrients, high biological production, and expanding oxygen minimum zones. The region acts as sink and source for atmospheric CO2. However, the efficiency of the biological carbon pump to sequester atmospheric CO2 and export particulate organic carbon from the surface is not well known. To quantify the upper ocean carbon export flux and to estimate the efficiency of biological carbon pump in the Bay of Bengal and the Indian Ocean, seawater profiles of total 234Th were measured from surface to 300 m depth at 13 stations from 19.9°N to 25.3°S in a transect along 87°E, during spring intermonsoon period (March–April 2014). Results showed enhanced in situ primary production in the equatorial Indian Ocean and the central Bay of Bengal and varied from 13.2 to 173.8 mmol C m−2 d−1. POC export flux in this region varied from 0 to 7.7 mmol C m−2 d−1. Though high carbon export flux was found in the equatorial region, remineralization of organic carbon in the surface and subsurface waters considerably reduced organic carbon export in the Bay of Bengal. Annually recurring anticyclonic eddies enhanced organic carbon utilization and heterotrophy. Oxygen minimum zone developed due to stratification and poor ventilation was intensified by subsurface remineralization. 234Th‐based carbon export fluxes were not comparable with empirical statistical model estimates based on primary production and temperature. Region‐specific refinement of model parameters is required to accurately predict POC export fluxes.

Role of zooplankton in determining the efficiency of the biological carbon pump

Abstract. The efficiency of the ocean's biological carbon pump (BCPeff – here the product of particle export and transfer efficiencies) plays a key role in the air–sea partitioning of CO2. Despite its importance in the global carbon cycle, the biological processes that control BCPeff are poorly known. We investigate the potential role that zooplankton play in the biological carbon pump using both in situ observations and model output. Observed and modelled estimates of fast, slow, and total sinking fluxes are presented from three oceanic sites: the Atlantic sector of the Southern Ocean, the temperate North Atlantic, and the equatorial Pacific oxygen minimum zone (OMZ). We find that observed particle export efficiency is inversely related to primary production likely due to zooplankton grazing, in direct contrast to the model estimates. The model and observations show strongest agreement in remineralization coefficients and BCPeff at the OMZ site where zooplankton processing of particles in the mesopelagic zone is thought to be low. As the model has limited representation of zooplankton-mediated remineralization processes, we suggest that these results point to the importance of zooplankton in setting BCPeff, including particle grazing and fragmentation, and the effect of diel vertical migration. We suggest that improving parameterizations of zooplankton processes may increase the fidelity of biogeochemical model estimates of the biological carbon pump. Future changes in climate such as the expansion of OMZs may decrease the role of zooplankton in the biological carbon pump globally, hence increasing its efficiency.