Glacial-interglacial productivity in the Polar Frontal Zone, southwest Pacific Ocean

<p>The Southern Ocean has a central role in regulating global climate change. Research has shown evidence of changes in biological productivity are coincident with increased iron deposition and rising atmospheric CO2 concentrations. The current data suggests these processes occur homogenously throughout the Southern Ocean, where research largely focuses on changes in biogenic silica as a proxy for upwelling and enhanced opal production. The role of calcium carbonate productivity, however, is rarely discussed, or is referred to in terms of preservation changes associated with shoaling and deepening of the lysocline. This assumption ignores potentially important effects of carbonate productivity and inter-basin complexities on ocean-atmosphere CO2 exchange. Two gravity cores (TAN1302-96 and TAN1302-97) collected from the southwest Pacific Polar Frontal Zone (PFZ) provide more insight into productivity changes and inter-basin differences across glacial-interglacial timescales. Detailed geochemical analysis, together with δ18O stratigraphy and 14C chronology, were used to reconstruct glacial-interglacial changes in terrigenous input and paleoproductivity in the PFZ. Sedimentological and biological analyses provide additional information to support the geochemical observations. This study highlights two distinct productivity modes (i.e. biogenic silica and calcium carbonate) that vary over glacial-interglacial timescales and with respect to the position of the Polar Front (PF). Key findings include; 1) a systematic series of key biological changes are repeated during glacial Terminations I (TI) and II (TII), the order of which depends on the position relative to the PF; 2) calcium carbonate productivity dominates the early part of the Termination north of the PF, whereas the production of biogenic silica dominates the early Termination south of the PF; 3) following TI and TII, calcium carbonate leads productivity in the early interglacials (i.e. MIS 5e and the Holocene), followed by the production of biogenic silica during the late interglacials, concurrent with declining atmospheric CO2 concentrations.</p>

Glacial-interglacial productivity in the Polar Frontal Zone, southwest Pacific Ocean

<p>The Southern Ocean has a central role in regulating global climate change. Research has shown evidence of changes in biological productivity are coincident with increased iron deposition and rising atmospheric CO2 concentrations. The current data suggests these processes occur homogenously throughout the Southern Ocean, where research largely focuses on changes in biogenic silica as a proxy for upwelling and enhanced opal production. The role of calcium carbonate productivity, however, is rarely discussed, or is referred to in terms of preservation changes associated with shoaling and deepening of the lysocline. This assumption ignores potentially important effects of carbonate productivity and inter-basin complexities on ocean-atmosphere CO2 exchange. Two gravity cores (TAN1302-96 and TAN1302-97) collected from the southwest Pacific Polar Frontal Zone (PFZ) provide more insight into productivity changes and inter-basin differences across glacial-interglacial timescales. Detailed geochemical analysis, together with δ18O stratigraphy and 14C chronology, were used to reconstruct glacial-interglacial changes in terrigenous input and paleoproductivity in the PFZ. Sedimentological and biological analyses provide additional information to support the geochemical observations. This study highlights two distinct productivity modes (i.e. biogenic silica and calcium carbonate) that vary over glacial-interglacial timescales and with respect to the position of the Polar Front (PF). Key findings include; 1) a systematic series of key biological changes are repeated during glacial Terminations I (TI) and II (TII), the order of which depends on the position relative to the PF; 2) calcium carbonate productivity dominates the early part of the Termination north of the PF, whereas the production of biogenic silica dominates the early Termination south of the PF; 3) following TI and TII, calcium carbonate leads productivity in the early interglacials (i.e. MIS 5e and the Holocene), followed by the production of biogenic silica during the late interglacials, concurrent with declining atmospheric CO2 concentrations.</p>

A missing link in the carbon cycle: phytoplankton light absorption

Marine biota and biogeophysical mechanisms, such as phytoplankton light absorption, have attracted increasing attention in recent climate studies. Under global warming, the impact of phytoplankton on the climate system is expected to change. Previous studies analyzed the impact of phytoplankton light absorption under prescribed future atmospheric CO2 concentrations. However, the role of this biogeophysical mechanism under freely-evolving atmospheric CO2 concentration and future CO2 emissions remain unknown. To shed light on this research gap, we perform simulations with the EcoGEnIE Earth system model and prescribe CO2 emissions following the four Representative Concentration Pathways (RCP) scenarios. Under all the RCP scenario, our results indicate that phytopankton light absorption increases the surface chlorophyll biomass, the sea surface temperature, the atmospheric CO2 concentrations and the atmospheric temperature. Under the RCP2.6, RCP4.5 and RCP6.0 scenarios, the magnitude of changes due to phytoplankton light absorption are similar. However, under the RCP8.5 scenario, the changes in the climate system are less pronounced due to the temperature limitation of phytoplankton growth, highlighting the reduced effect of phytoplankton light absorption under strong warming. Additionally, this work evidences the major role of phytoplankton light absorption on the climate system, suggesting a highly uncertain feedbacks on the carbon cycle with uncertainties that are in the range of those known from the land biota.

Elevated CO2 Concentration Alters Photosynthetic Performances under Fluctuating Light in Arabidopsis thaliana

In view of the current and expected future rise in atmospheric CO2 concentrations, we examined the effect of elevated CO2 on photoinhibition of photosystem I (PSI) under fluctuating light in Arabidopsis thaliana. At 400 ppm CO2, PSI showed a transient over-reduction within the first 30 s after transition from dark to actinic light. Under the same CO2 conditions, PSI was highly reduced after a transition from low to high light for 20 s. However, such PSI over-reduction greatly decreased when measured in 800 ppm CO2, indicating that elevated atmospheric CO2 facilitates the rapid oxidation of PSI under fluctuating light. Furthermore, after fluctuating light treatment, residual PSI activity was significantly higher in 800 ppm CO2 than in 400 ppm CO2, suggesting that elevated atmospheric CO2 mitigates PSI photoinhibition under fluctuating light. We further demonstrate that elevated CO2 does not affect PSI activity under fluctuating light via changes in non-photochemical quenching or cyclic electron transport, but rather from a rapid electron sink driven by CO2 fixation. Therefore, elevated CO2 mitigates PSI photoinhibition under fluctuating light at the acceptor rather than the donor side. Taken together, these observations indicate that elevated atmospheric CO2 can have large effects on thylakoid reactions under fluctuating light.

The Impact of Photorespiratory Glycolate Oxidase Activity on Arabidopsis thaliana Leaf Soluble Amino Acid Pool Sizes during Acclimation to Low Atmospheric CO2 Concentrations

Photorespiration is a metabolic process that removes toxic 2-phosphoglycolate produced by the oxygenase activity of ribulose-1,5-bisphosphate carboxylase/oxygenase. It is essential for plant growth under ambient air, and it can play an important role under stress conditions that reduce CO2 entry into the leaf thus enhancing photorespiration. The aim of the study was to determine the impact of photorespiration on Arabidopsis thaliana leaf amino acid metabolism under low atmospheric CO2 concentrations. To achieve this, wild-type plants and photorespiratory glycolate oxidase (gox) mutants were given either short-term (4 h) or long-term (1 to 8 d) low atmospheric CO2 concentration treatments and leaf amino acid levels were measured and analyzed. Low CO2 treatments rapidly decreased net CO2 assimilation rate and triggered a broad reconfiguration of soluble amino acids. The most significant changes involved photorespiratory Gly and Ser, aromatic and branched-chain amino acids as well as Ala, Asp, Asn, Arg, GABA and homoSer. While the Gly/Ser ratio increased in all Arabidopsis lines between air and low CO2 conditions, low CO2 conditions led to a higher increase in both Gly and Ser contents in gox1 and gox2.2 mutants when compared to wild-type and gox2.1 plants. Results are discussed with respect to potential limiting enzymatic steps with a special emphasis on photorespiratory aminotransferase activities and the complexity of photorespiration.

Agricultural Greenhouse Gas Emissions in a Data-Scarce Region Using a Scenario-Based Modeling Approach: A Case Study in Southeastern USA

Climate change may impact agricultural greenhouse gas emissions (GHGs) and yields under higher temperatures, higher atmospheric CO2 concentrations, and variable precipitations. This calls for adaptation strategies to optimize agricultural productions with minimal GHGs. This study aimed to identify these optimum agricultural managements in response to current and projected climatic scenarios for the Choctawhatchee Basin in Southeastern USA, an experimentally unexplored data-scarce region lacking validation data. This scenario-based modeling study analyzed a total of 1344 scenarios consisting of four major crops, eight managements (varying tillage, manuring, and residue), and forty climatic combinations under current as wells as two representative concentration pathways with process-based Denitrification and Decomposition (DNDC) model. The results indicated that the region’s GHGs and yields were most affected by higher temperatures (≥+3 °C) and extreme precipitation changes (≥±40%), while high atmospheric CO2 concentrations exerted positive fertilization effects. The manure-related and higher residue incorporation scenarios were found to be better options in varying climates with minimal present global warming potentials (GWP) of 0.23 k to −29.1 k MT equivalent CO2. As such, the study presented climate change impacts and potential mitigation options in the study region while presenting a framework to design GHG mitigation in similar data-scarce regions.

Parallel between the isotopic composition of coccolith calcite and carbon levels across Termination II: Developing a new paleo-CO₂ probe

Beyond the pCO2 records provided by ice core measurements, the quantification of atmospheric CO2 concentrations and changes thereof relies on proxy data, the development of which represents a foremost challenge in paleoceanography. In the paleoceanographic toolbox, the coccolithophores occupy a notable place, as the magnitude of the carbon isotopic fractionation between ambient CO2 and a type of organic compounds that these photosynthetic microalgae synthesize (the alkenones) represents a relatively robust proxy to reconstruct past atmospheric CO2 concentrations during the Cenozoic. The isotopic composition of coeval calcite biominerals found in the sediments and also produced by the coccolithophores (the coccoliths) have been found to record an ambient CO2 signal through culture and sediment analyses. These studies have, however, not yet formalized a transfer function that quantitatively ties the isotopic composition of coccolith calcite to the concentrations of aqueous CO2, and, ultimately, to atmospheric CO2 levels. Here, we make use of a micro-separation protocol to compare the isotopic response of two size-restricted coccolith assemblages from the North Atlantic to changes in surface ocean CO2 during Termination II (ca. 130–140 ka). Performing paired measurements of the isotopic composition (δ13C and δ18O) of relatively large and small coccoliths provides an isotopic offset that can be designated as a “differential vital effect”. We find that the evolution of this offset follows that of aqueous CO2 concentrations computed from the ice core CO2 curve and an independent temperature signal. We interpret this biogeochemical feature to be the result of converging carbon fixation strategies between large and small cells as the degree of carbon limitation for cellular growth decreases across the deglaciation. We are therefore able to determine a transfer function between the coccolith differential vital effects and aqueous CO2 in the range of Quaternary CO2 concentrations. We here consolidate a new coccolith ∆δ13C proxy that overtakes the strong assumptions that have to be made pertaining to the chemistry of the carbonate system in seawater, as required in CO2 proxy methods such as the boron isotope and alkenone proxies.