Atmospheric pCO2 Research Articles

Carbon Cycle and Circulation Change in the North Pacific Ocean at the Initiation of Northern Hemisphere Glaciation Constrained by Boron‐Based Proxies in Diatoms

AbstractThe intensification of Northern Hemisphere glaciation (iNHG) at 2.73 Ma is associated with a reorganization of the subarctic Pacific Ocean and abrupt drop in opal mass accumulation rates. Uncertainty, however, remains around the extent to which these changes altered carbon dynamics and contributed to a reduction in atmospheric pCO2 and global temperatures. These issues are addressed here using the boron isotope (δ11B) proxy in diatom frustules to reconstruct past changes in the pH and pCO2 of ambient seawater. Diatom δ11B and [B] indicate a subarctic Pacific surface water increase of 0.3–0.5 pH units over the iNHG. This confirms that delivery of carbon and nutrients into surface waters was reduced at this time, explaining the drop in opal productivity and limiting CO2 outgassing from the ocean interior. We consider two hypotheses to explain this based on potential changes in circulation from the late Pliocene to early Pleistocene: “ventilation to stratification” or “stratification to ventilation.” The ventilation to stratification hypothesis, which posits a switch from vigorous Pacific Meridional Overturning Circulation in the Pliocene to stratification over iNHG, has received more attention in the literature. The stratification to ventilation hypothesis, which posits a modest increase in ventilation, is more consistent with modern and late Pleistocene analogs, the majority of models and δ13C data. These late Pliocene changes in the subarctic Pacific, in conjunction with other external and internal processes including those in the Southern Ocean, would have contributed to a lowering of atmospheric pCO2 and the long‐term expansion of ice‐sheets across the Northern Hemisphere.

Distribution and air-sea fluxes of CO2 in coral reefs in the Greater Bay Area, China

From May to June 2021, the sea surface partial pressure of CO2 (pCO2) and air-sea CO2 fluxes around six coral reefs in the Great Bay Area (contain Pearl River Estuary (PRE) and Daya Bay) were investigated respectively. The distribution of seawater pCO2 around different coral reefs were quite different. The seawater pCO2 of the coral reef region of Daya Bay is relatively high, ranging from 268 to 496 µatm. In contrast, the pCO2 levels in the coral reef area of the PRE are significantly lower, with measurements ranging from 84 to 374 µatm. Further analysis showed that the distribution of seawater pCO2 around the coral reefs of the PRE were mainly affected by biological metabolism and seawater mixing, while those in Daya Bay were mainly controlled by temperature and seawater mixing. During the survey, the three coral reefs in the PRE were affected by algal blooms showing strong sinks of atmospheric CO2 ranging from −12.3 to −19.2 mmol CO2 m−2 d−1. However, the difference between the seawater pCO2 and atmospheric pCO2 in the three coral reefs of Daya Bay were all very small, and they were weak sources or sinks of atmospheric CO2, respectively.

Diverse glacial ventilation in deep Pacific: An integrated record from Mariana Trench and Magellan Seamounts over the last 1.2 Myr [Global and Planetary Change, volume 230, November 2023, 104279]: Comment

Yi (2023) proposed a new deep Western Pacific ventilation hypothesis using beryllium (10Be/9Be) and 230Thex dates, magnetostratigraphy, and element-based tuning data from two box cores and one gravity core. However, given the low sedimentation rates of these cores and uncertain stratigraphy, we suggest that some interpretations require further clarifications and corrections. The low sedimentation rates cast doubt on the validity of the millennial-scale western Pacific bottom-water oxygenation variability. The lack of unsupported explanations about the newly found periodicity in the Mariana Trench and Magellan Seamounts records raises concern about the confidence of the age models. The phasing between the North Atlantic deep water and Antarctic bottom water formation and atmospheric pCO2 in the deep Western Pacific setting is unclear.

Were Late Jurassic climatic fluctuations responses to Pangea breakup? Evidence from isotopic analyses of belemnite rostra from the eastern Tethyan Ocean

Late Jurassic paleogeographic patterns were influenced by the growth of the Pacific plate and the breakup of Pangea, impacting the global paleoclimate during this warm and equable greenhouse period. However, the use of different proxies may introduce bias in seawater temperature reconstructions. In this study, we reconstructed midlatitude Tethyan seawater paleotemperatures in the Northern Hemisphere using carbonate clumped isotopes (Δ47) in well-preserved parts of belemnite rostra from the Upper Jurassic Hongqilafu Formation in the Taxkorgan Basin, southwest Xinjiang, China. Throughout the entire studied section, an increase in the 87Sr/86Sr ratio was observed. Comparison of this ratio with the published global seawater 87Sr/86Sr curve suggests a good correlation with the biostratigraphic and U-Pb age assignments of the Oxfordian−Tithonian interval. The Δ47-derived paleotemperature estimates indicate a long-term, relatively stable, and warm seawater temperature of ∼27 °C within the epipelagic zone of the eastern Tethyan Ocean during the Late Jurassic. Cooler seawater temperatures were recorded at the Oxfordian−Kimmeridgian boundary and in the late Tithonian, likely reflecting changes in paleogeography and paleoceanography resulting from the breakup of Pangea and/or variations in belemnite habitat depth. When compared with global seawater temperature data from different proxies, the results suggest that elevated atmospheric pCO2 levels during the Late Jurassic may have caused warmer conditions in midlatitude and polar regions. The reconstructed δ18Osea values exhibit a remarkably modern midlatitude seawater character and are more positive than values traditionally assumed for the Jurassic ice-free world. Given that the Taxkorgan Basin was located in a semi-enclosed basin and was dominated by evaporation in the Late Jurassic, the reconstructed δ18Osea values may represent regional seawater signals. Consequently, they cannot be used to support the existence of ice sheets in both polar regions. Our results suggest that climatic changes in the Jurassic greenhouse world may have been overprinted by local factors, potentially masking broader climatic trends.

Stable carbon isotope ratios of pristine carbohydrates preserved within nannofossil calcite

The geochemical characterization of phytoplankton-derived organic compounds found in marine sediments has been widely used to reconstruct atmospheric pCO2 throughout the Cenozoic. This is possible owing to a well-established relationship between the carbon isotope ratios of phytoplankton biomass and CO2 concentration in the ambient seawater. An ideal molecular target for such proxy reconstructions would be degradation resistant on geologic timescales and unambiguously associated with known, experimentally tractable, organisms, so that species-specific models can be developed, calibrated, and applied to appropriate material. However, existing organic matter targets do not meet these criteria, primarily owing to ambiguity in the source species of recalcitrant compounds in deep time. Here we explore the potential of a novel organic carbon target for isotopic analysis: acidic polysaccharides extracted from the calcite plates (coccoliths) that are produced by all calcifying haptophytes. Carbohydrates are usually rapidly remineralized in sediments, but coccolith-associated polysaccharides (CAPs) are mechanically protected from diagenesis within the coccolith calcite lattice. Coccoliths can be taxonomically separated by size and identified, often to species level, prior to CAP extraction, providing a species-specific record. Coccolith morphology and composition are important additional sources of information, which are then unambiguously associated with the extracted CAPs. We found that carbon isotope ratios of CAPs changed in response to the environmental changes associated with a glacial cycle, which we attribute to temperature-driven changes in average growth rate. Once the underlying biosynthetic processes and the associated isotope effects are better understood, this archive of pristine organic matter has the potential to provide insight into phytoplankton growth rates and atmospheric pCO2 far beyond the Cenozoic, to when the first coccolithophores inhabited the surface ocean over 200 million years ago.

Carbon Dioxide Flux and its Relationship to Water Quality in the Teluk Awur - Jepara, Indonesia

Abstract Anthropogenic activities on land will affect the carbon system in coastal waters. This condition will affect the role of coastal waters as a source or sink of carbon. This research will examine the distribution of carbonate systems and estimate CO2 fluxes. Water samples were taken at 30 stations as well as measuring in situ water parameters including pH, temperature, salinity, and pressure. Alkalinity was analysed based on the titration method, chl-a using the fluorometer method, and carbonate system parameters (pCO2(sea) and DIC) calculated using CO2SYS.xlsm. The pCO2(atm) value is calculated based on atmospheric data from satellite data, including the saturation vapour pressure of sea air in the atmosphere, fraction mol of CO2, and sea level pressure. Pearson correlation analysis was used to see the correlation between the measured parameters and CO2 flux. Carbon flux is determined based on the difference between atmospheric and oceanic pCO2. The DIC calculation result was in the range of 1,946.09 µmol/kg - 2,061.65 µmol/kg and the total alkalinity was 2,156.86 µmol/kg - 2,264.71 µmol/kg. Based on this value, Jepara coastal waters have pCO2(sea) of 573,800 micro-atmospheric (µatm) which is higher than pCO2(atm) (i.e., 386,772 µatm). The input of organic/inorganic carbon resulting from anthropogenic processes on land has influenced Jepara coastal waters which act as a source of CO2 into the atmosphere amounting to 103,799 mmol/m2/day. The results of this research can be used as a reference for managing coastal areas to achieve low carbon emissions.

The Campanian-Maastrichtian benthic foraminiferal assemblages at the Elles section (Tunisia): A perspective for paleoenvironmental, paleobathymetric and sea-level fluctuation reconstruction

The Late Cretaceous Period experienced prevailing hothouse conditions that were characterized by relatively elevated atmospheric pCO2 concentrations, high global mean sea level and mostly lacking ice sheets. These conditions were interrupted by an overall global decreasing trend in temperature during the Campanian-Maastrichtian interval (83.65 to 66.04 Ma). The Upper Cretaceous sedimentary successions in Tunisia (southwestern Tethyan Ocean) like at the Elles section are regarded as unique archives for investigating paleoenvironmental and paleoclimatic changes, as well as constrain oscillations of sea-level. Here, we perform high resolution quantitative analysis of the benthic foraminiferal assemblages in the Campanian-Maastrichtian interval at the Elles section in order to infer palaeoenvironmental and paleobathymetric changes in relative sea-level. The Campanian succession (Abiod Fm.) is inferred to be deposited at upper to middle bathyal (i.e., 540–675 m) and is characterized by prevailing well‑oxygenated conditions and low organic matter flux to the seafloor. The Campanian/Maastrichtian transition-coincides with the lowermost part of the El Haria Fm. deposited in outer neritic to upper bathyal settings under oligotrophic, well‑oxygenated and reduced productivity conditions. Shallower depositional setting (i.e., 436–515 m) is inferred for the Maastrichtian succession (El Haria Fm.) that is also characterized by a lowering in oxygen availability at the seafloor associated with an increase of organic matter. Eight main sea-level regressions (four in the Campanian, one at the Campanian-Maastrichtian transition and three in the Maastrichtian) have been identified and biostratigraphically constrained. These sea-level regression events are mostly accompanied by a negative excursion carbon isotope (δ13C) and their stratigraphical positions appear to be quite consistent with those previously recognized at the nearby El Kef section.

Volcanic and orbitally forced carbon release during the Middle Eocene Climatic Optimum

The drivers of the Middle Eocene Climatic Optimum (MECO) remain enigmatic. Here we report a high-resolution terrestrial MECO record from the Bohai Bay Basin, eastern China. The record shows episodic Hg enrichments and a volcanogenic Δ199Hg signature during the MECO, and an abrupt chemical weathering enhancement and negative δ13Corg excursion (CIE) during the MECO peak warmth. A high-resolution age model constrains the MECO to ca. 40.59−40.18 Ma with the onset in the first ∼320 k.y. Peak warmth and the CIE initiated at 40.27 Ma, corresponding to 405 k.y. and 100 k.y. eccentricity maxima, and lasted for ∼90 k.y., reminiscent of the early Eocene hyperthermals. Our findings suggest that episodic volcanism contributed to gradual atmospheric pCO2 rise, leading to the long duration of the MECO onset. Orbital forcing at 40.27 Ma, on top of CO2 forcing, caused the peak warmth and a positive carbon cycle feedback. The regional increase in weathering is inconsistent with evidence for weakened global continental weathering, suggesting strong spatial heterogeneity in the weathering response to MECO warming.

Boron isotopic compositions of middle Miocene to recent shallow-water carbonates from the South China Sea: Assessing diagenetic effects and implications for paleoclimate changes

The partial pressure of atmospheric CO2 (pCO2) significantly influences global climate change and biological evolution through geological history. Boron isotopic composition (δ11B) in carbonates has been used to reconstruct the paleo-pH of seawater, providing insight into atmospheric pCO2 levels. However, the fidelity of δ11B records in marine carbonates due to diagenesis remains uncertain. Here, to understand how diagenetic processes influence B isotopic records in marine carbonates, we examined B concentrations and δ11B values of modern corals (from Hainan Island), unconsolidated shallow pushcores (Jiuzhang A and Jiuzhang B from Nansha Islands, South China Sea), and a long drillcore (XK-1 from Xisha Islands, South China Sea) covering the Late Miocene to Holocene periods. Our coral samples show a uniform δ11B range (20.16 ± 0.85%), consistent with previously published values for modern corals in other open oceans. The δ11B values of the unlithified carbonate sediments from Jiuzhang A and B pushcores vary within a narrow range (18.70 ± 0.84%), yielding pH and CO2 concentrations consistent with the range between modern and pre-industrial values. Since we did not observe any statistically significant covariations between traditionally established diagenetic proxies (such as δ13C, δ18O, Mn/Sr, and Al/Ca) and δ11B values for non-dolomitized samples in the XK-1 drillcore, we suggest that δ11B values of bulk limestones are not significantly affected by typical diagenesis (meteoric, mixed-diagenetic, and marine diagenetic processes), likely due to limited post-depositional recrystallization of our study carbonates. In contrast, dolomitization significantly decreases δ11B values of bulk carbonates, rendering dolomites unsuitable as archives for reconstructing seawater δ11Bborate values. Our study supports that marine limestone with limited recrystallization or dolomitization have the potential to record ambient seawater pH values with high resolution. The established secular seawater pH variations based on XK-1 δ11B records provide information about ocean acidification over the past 5.1 million years.

Persistent cooling in the Ordovician (Darriwilian–Sandbian) revealed by conodont δ18O records in the Tarim Basin, NW China: Climatic and sedimentary implications

The Great Ordovician Biodiversification Event (GOBE) was considered to be primarily triggered by climate cooling. However, the precise timing of the greenhouse-icehouse transition during the Ordovician Period remains largely unconstrained. In order to reconstruct palaeotemperatures during the Middle−Late Ordovician transition in the Tarim Basin, northwestern China, a high-resolution SIMS-based oxygen isotope (δ18Oapatite) profile was obtained from 65 conodont elements from the Nanyigou section. The oxygen isotope record based on mono-generic samples reveals an increase in δ18Oapatite by 1.6‰ (VSMOW) during the late Darriwilian to Sandbian, translating to tropical sea-surface water cooling of ∼7 °C, probably marking the initiation of polar ice sheet growth and the prelude of the Hirnantian maximum. This δ18Oapatite shift agrees well with the time-equivalent δ18Oapatite records from other continents, thus suggestiing a global signal. Furthermore, the positive shift of δ18Oapatite overall coincides with inorganic carbon isotope (δ13Ccarb) positive excursion, indicating that enhanced organic carbon burial may have led to the atmospheric pCO2 drawdown. Additionally, the temporal coincidence between the δ18Oapatite positive excursion and development of the purplish-red nodular limestone of the Tumuxiuke Formation suggests that global cooling may have contributed to the formation of coeval marine red beds.

Preliminary investigation on the potential involvement of an ABC-like gene in Halomicronema metazoicum (Cyanobacteria) tolerance to low seawater pH in an ocean acidification scenario

Decreasing ocean surface pH, called ocean acidification (OA), is among the major risks for marine ecosystems due to human-driven atmospheric pCO2 increase. Understanding the molecular mechanisms of adaptation enabling marine species to tolerate a lowered seawater pH could support predictions of consequences of future OA scenarios for marine life. This study examined whether the ATP-binding cassette (ABC)-like gene slr2019 confers tolerance to the marine cyanobacterium Halomicronema metazoicum to low seawater pH conditions (7.7, 7.2, 6.5) in short- and long-term exposures (7 and 30 d). Photosynthetic pigment content indicated that the species can tolerate all three lowered-pH conditions. At day 7, slr2019 was up-regulated at pH 7.7 while no changes were observed at lower pH. After 30-d exposure, a significant decrease in slr2019 transcript levels was observed in all low-pH treatments. These first results indicate an effect of low pH on the examined transporter expression in H. metazoicum.

Atmospheric pCO2 Response to Stimulated Organic Carbon Export: Sensitivity Patterns and Timescales

AbstractThe ocean's organic carbon export is a key control on atmospheric pCO2 and stimulating this export could potentially mitigate climate change. We use a data‐constrained model to calculate the sensitivity of atmospheric pCO2 to local changes in export using an adjoint approach. A perpetual enhancement of the biological pump's export by 0.1 PgC/yr could achieve a roughly 1% reduction in pCO2 at average sensitivity. The sensitivity varies roughly 5‐fold across different ocean regions and is proportional to the difference between the mean sequestration time τseq of regenerated carbon and the response time τpre of performed carbon, which is the reduction in the preformed carbon inventory per unit increase in local export production. Air‐sea CO2 disequilibrium modulates the geographic pattern of τpre, causing particularly high sensitivities (2–3 times the global mean) in the Antarctic Divergence region of the Southern Ocean.

Productivity in the Southern Ocean Antarctic Zone during the Northern Hemisphere Glaciation (NHG) and its link to atmospheric pCO2

Productivity in the Southern Ocean Antarctic Zone during the Northern Hemisphere Glaciation (NHG) and its link to atmospheric pCO2

Changes in pCO2 and climate paced by grand orbital cycles in the late Cenozoic

As one of the most important greenhouse gases, CO2 is considered a major controlling factor of Earth's climate over geological timescales. However, the origins of quasi-periodic fluctuations in pCO2 on a million-year timescale remain unclear. Here, we used published datasets of atmospheric pCO2, oxygen isotopes of benthic foraminifera (δ18Obenthic) and global mean sea-level (GMSL) from 23 Ma to the present to explore the pacing of pCO2 changes and concomitant climatic effects using multiple time series analysis approaches. Our results indicate that the evolution of late Cenozoic pCO2 and climate was paced by the grand orbital cycles, in particular the ∼4.5 Myr and ∼2.4 Myr eccentricity cycles, and ∼1.3 Myr obliquity cycle. Periodic occurrence of cold conditions was associated with low climate seasonality during the minima of ∼4.5 Myr and ∼2.4 Myr eccentricity cycles. We suggest that cooler conditions are associated with decreased atmospheric pCO2 as a result of higher organic carbon burial due to lower metabolic rate of heterotrophic bacteria and more organic carbon export to the deep ocean. Furthermore, the buildup of glaciers during the minima of grand eccentricity cycles might lower pCO2 via increased ice cover and enhanced dust fluxes. In contrast, high seasonal climate may lead to an opposite effect on atmospheric pCO2 during the maxima of the grand eccentricity cycles. Moreover, we found a distinct shift in the dominant signal from eccentricity to obliquity cycles recorded in the pCO2, δ18Obenthic and GMSL datasets at ∼13 Ma, a time when perennial sea ice occurred in the Arctic and significant ice growth shown in Antarctica. We suggest that the change in the type and distribution of the ice sheets would shift glacial response to orbital forcing and hence mediated global climate and pCO2. Our analysis reveals a clear synchrony among atmospheric pCO2, climate change, and the grand orbital cycles in the late Cenozoic.

Response of atmospheric pCO2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_2$$\\end{document} to a strong AMOC weakening under low and high emission scenarios

The Earth System is warming due to anthropogenic greenhouse gas emissions which increases the risk of passing a tipping point in the Earth System, such as a collapse of the Atlantic Meridional Overturning Circulation (AMOC). An AMOC weakening can have large climate impacts which influences the marine and terrestrial carbon cycle and hence atmospheric pCO2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_2$$\\end{document}. However, the sign and mechanism of this response are subject to uncertainty. Here, we use a state-of-the-art Earth System Model, the Community Earth System Model v2 (CESM2), to study the atmospheric pCO2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_2$$\\end{document} response to an AMOC weakening under low (SSP1-2.6) and high (SSP5-8.5) emission scenarios over the years 2015–2100. A freshwater flux anomaly in the North Atlantic strongly weakens the AMOC, and we simulate a weak positive pCO2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_2$$\\end{document} response of 0.45 and 1.3 ppm increase per AMOC decrease in Sv for SSP1-2.6 and SSP5-8.5, respectively. For SSP1-2.6 this response is driven by both the oceanic and terrestrial carbon cycles, whereas in SSP5-8.5 it is solely the ocean that drives the response. However, the spatial patterns of both the climate and carbon cycle response are similar in both emission scenarios over the course of the simulation period (2015–2100), showing that the response pattern is not dependent on cumulative CO2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_2$$\\end{document} emissions up to 2100. Though the global atmospheric pCO2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_2$$\\end{document} response might be small, locally large changes in both the carbon cycle and the climate system occur due to the AMOC weakening, which can have large detrimental effects on ecosystems and society.

Stream CO2 emissions are overestimated without consideration of diel water and atmospheric CO2 variability

AbstractStreams are significant emitters of carbon dioxide (CO2) to the atmosphere that are influenced by diel CO2 dynamics. However, we know little about diel CO2 variability within streams, the diel dynamics of CO2 in the air above streams, and the consequences for emission calculations. We studied five pre‐alpine streams by equipping three sites per stream in close proximity (~ 1 km apart) with automatic logging stations that continuously recorded water and air CO2 partial pressures (pCO2) for 2–4 d. All streams and sites showed increased pCO2 at night and decreased pCO2 during the day, however, with fourfold higher diel amplitudes for atmospheric pCO2 compared to the water. Calculating diffusive CO2 fluxes with fixed compared to dynamic measured atmospheric CO2 resulted in negligible to 431% lower estimates. We might thus currently overestimate fluvial CO2 emissions and should include diel water and air CO2 variability to more accurately assess stream CO2 emissions.

Underway pCO2 Surveys Unravel CO2 Invasion of Lake Superior From Seasonal Variability

AbstractThis study observed seasonal trends and inferred drivers of CO2 biogeochemistry at the air‐water interface of Lake Superior. Underway carbon dioxide partial pressure (pCO2) was measured in surface water during 69 transects spanning ice free seasons of 2019‐2022. These data comprise the first multiannual pCO2 time series in the Laurentian Great Lakes. Surface water pCO2 was closely tied to increasing atmospheric pCO2 over a 100‐day CO2 equilibration timescale, while seasonal variability was controlled equally by thermal and non‐thermal drivers during the ice‐free season. Comparison to previous modeling efforts indicated that Lake Superior surface pCO2 increased with two decades of rising atmospheric CO2. Spatial heterogeneity in CO2 dynamics was highlighted by a conductivity‐based delineation of “riverine” and “pelagic” regimes, each of which was associated with net CO2 influx over Julian days 100–300 on the order of 25 Gmol C. These findings refine previous estimates of Lake Superior C fluxes, support predictions of anthropogenic CO2 invasion during the ice‐free season, point to new observation strategies for large lakes, and highlight an urgent need for studies of changes to lacustrine C cycling.

Alkenone-derived estimates of Cretaceous pCO2

Alkenones are long-chain ketones produced by phytoplankton of the order Isochrysidales. They are widely used in reconstructing past sea surface temperatures, benefiting from their ubiquitous occurrence in the Cenozoic ocean. Carbon isotope fractionation (εp) between alkenones and dissolved inorganic carbon may also be used as a proxy for past atmospheric pCO2 and has provided continuous pCO2 estimates back to ca. 45 Ma. Here, an extended occurrence of alkenones from ca. 130 Ma is reported. We characterize the molecular structure and distribution of these Mesozoic alkenones and evaluate their potential phylogenetic relationship with Cenozoic alkenones. Using δ13C values of the C37 methyl alkenone (C37:2Me), the first alkenone-based pCO2 estimates for the Mesozoic are derived. These estimates suggest elevated pCO2 with a range of 548−4090 ppm (908 ppm median) during the super-greenhouse climate of the Early Cretaceous, in agreement with phytane-based pCO2 reconstructions. Finally, insights into the identity of the Cretaceous coccolithophores that possibly synthesized alkenones are also offered.

Assessing transient changes in the ocean carbon cycle during the last deglaciation through carbon isotope modeling

Abstract. Atmospheric carbon dioxide concentration (pCO2) has increased by approximately 80 ppm from the Last Glacial Maximum (LGM) to the early Holocene. The change in this atmospheric greenhouse gas is recognized as a climate system response to gradual change in insolation. Previous modeling studies suggested that the deglacial increase in atmospheric pCO2 is primarily attributed to the release of CO2 from the ocean. Additionally, it has been suggested that abrupt change in the Atlantic meridional overturning circulation (AMOC) and associated interhemispheric climate changes are involved in the release of CO2. However, understanding remains limited regarding oceanic circulation changes and the factors responsible for changes in chemical tracers in the ocean during the last deglaciation and their impact on atmospheric pCO2. In this study, we investigate the evolution of the ocean carbon cycle during the last deglaciation (21 to 11 ka BP) using three-dimensional ocean fields from the transient simulation of the MIROC 4m climate model, which exhibits abrupt AMOC changes similar to those observed in reconstructions. We investigate the reliability of simulated changes in the ocean carbon cycle by comparing the simulated carbon isotope ratios with sediment core data, and we examine potential biases and overlooked or underestimated processes in the model. Qualitatively, the modeled changes in atmospheric pCO2 are consistent with ice core records. For example, during Heinrich Stadial 1 (HS1), atmospheric pCO2 increases by 10.2 ppm, followed by a reduction of 7.0 ppm during the Bølling–Allerød (BA) period and then by an increase of 6.8 ppm during the Younger Dryas (YD) period. However, the model underestimates the changes in atmospheric pCO2 during these events compared to values derived from ice core data. Radiocarbon and stable isotope signatures (Δ14C and δ13C) indicate that the model underestimates both the activated deep-ocean ventilation and reduced efficiency of biological carbon export in the Southern Ocean and the active ventilation in the North Pacific Intermediate Water (NPIW) during HS1. The relatively small changes in simulated atmospheric pCO2 during HS1 might be attributable to these underestimations of ocean circulation variation. The changes in Δ14C associated with strengthening and weakening of the AMOC during the BA and YD periods are generally consistent with values derived from sediment core records. However, although the data indicate continuous increase in δ13C in the deep ocean throughout the YD period, the model shows the opposite trend. It suggests that the model either simulates excessive weakening of the AMOC during the YD period or has limited representation of geochemical processes, including marine ecosystem response and terrestrial carbon storage. Decomposing the factors behind the changes in ocean pCO2 reveals that variations in temperature and alkalinity have the greatest impact on change in atmospheric pCO2. Compensation for the effects of temperature and alkalinity suggests that the AMOC changes and the associated bipolar climate changes contribute to the decrease in atmospheric pCO2 during the BA and the increase in atmospheric pCO2 during the YD period.

ILOSCAR: interactive Long-term Ocean-atmosphere-Sediment CArbon cycle Reservoir model v1.0

Computationally inexpensive carbon cycle models serve as critical and efficient tools for illuminating the complex dynamics of the carbon cycle and its interplay with the climate system, offering insights into how our planet has responded to climate perturbations throughout its history. During geologic hyperthermal events, carbon cycle models are employed to trace the trajectory of carbon emissions and establish a connection between the emission trajectory and changes in the Earth's surface environment. To date, the prevalent method to estimate the carbon emission rate relies on coupling the carbon cycle modeling and proxy reconstructions. Most previous studies employ a forward methodology, i.e., they force the model with an array of predefined carbon injection scenarios and select the one that produces the best fit to one or more specific proxy-derived records (e.g., atmospheric pCO2, sea surface pH, and calcite compensation depth) as the optimal solution. However, this forward method has two potential disadvantages. First, it can be computationally expensive, particularly when tens of thousands of scenarios need to be conducted to find the best solution. Second, it might not yield the best injection trajectory if none of the predefined carbon emission curves represents the realistic emission curve. Hence, an inverse model that can reconstruct the carbon emission trajectory directly from the record/records is urgently needed. In this study, building upon the Long-term Ocean-atmosphere-Sediment CArbon cycle Reservoir (LOSCAR) Model (Zeebe, 2012), we develop an interactive carbon cycle model (named iLOSCAR) using the open-source Python language and include two options, a forward model and an inverse model. The forward model replicates the original LOSCAR model, while the inverse model calculates the emission trajectory constrained by the proxy records in a single run. Both models are accessible via a web-based interface, which allows users to interactively tune model parameters and conduct experiments. In this paper, we present the details of iLOSCAR, including model structure and derivation of key equations. We then validate iLOSCAR's performance through an identical twin test and model intercomparison. We also apply it to a climatic perturbation event to diagnose features of the emission pattern that were overlooked in previous studies. Finally, we discuss the possible directions for model's future development.