Mass extinction at the Cretaceous–Paleogene (K-Pg) boundary coincides with the Chicxulub bolide impact and also falls within the broader time frame of Deccan trap emplacement. Critically, though, ...empirical evidence as to how either of these factors could have driven observed extinction patterns and carbon cycle perturbations is still lacking. Here, using boron isotopes in foraminifera, we document a geologically rapid surface-ocean pH drop following the Chicxulub impact, supporting impact-induced ocean acidification as amechanism for ecological collapse in the marine realm. Subsequently, surface water pH rebounded sharply with the extinction of marine calcifiers and the associated imbalance in the global carbon cycle. Our reconstructed water-column pH gradients, combined with Earth system modeling, indicate that a partial ∼50% reduction in global marine primary productivity is sufficient to explain observed marine carbon isotope patterns at the K-Pg, due to the underlying action of the solubility pump. While primary productivity recovered within a few tens of thousands of years, inefficiency in carbon export to the deep sea lasted much longer. This phased recovery scenario reconciles competing hypotheses previously put forward to explain the K-Pg carbon isotope records, and explains both spatially variable patterns of change in marine productivity across the event and a lack of extinction at the deep sea floor. In sum, we provide insights into the drivers of the last mass extinction, the recovery of marine carbon cycling in a postextinction world, and the way in which marine life imprints its isotopic signal onto the geological record.
The cause of the end-Cretaceous mass extinction is vigorously debated, owing to the occurrence of a very large bolide impact and flood basalt volcanism near the boundary. Disentangling their relative ...importance is complicated by uncertainty regarding kill mechanisms and the relative timing of volcanogenic outgassing, impact, and extinction. We used carbon cycle modeling and paleotemperature records to constrain the timing of volcanogenic outgassing. We found support for major outgassing beginning and ending distinctly before the impact, with only the impact coinciding with mass extinction and biologically amplified carbon cycle change. Our models show that these extinction-related carbon cycle changes would have allowed the ocean to absorb massive amounts of carbon dioxide, thus limiting the global warming otherwise expected from postextinction volcanism.
Pelagic ecosystem function is integral to global biogeochemical cycling, and plays a major role in modulating atmospheric CO2 concentrations (pCO2). Uncertainty as to the effects of human activities ...on marine ecosystem function hinders projection of future atmospheric pCO2. To this end, events in the geological past can provide informative case studies in the response of ecosystem function to environmental and ecological changes. Around the Cretaceous–Palaeogene (K–Pg) boundary, two such events occurred: Deccan large igneous province (LIP) eruptions and massive bolide impact at the Yucatan Peninsula. Both perturbed the environment, but only the impact coincided with marine mass extinction. As such, we use these events to directly contrast the response of marine biogeochemical cycling to environmental perturbation with and without changes in global species richness. We measure this biogeochemical response using records of deep-sea carbonate preservation. We find that Late Cretaceous Deccan volcanism prompted transient deep-sea carbonate dissolution of a larger magnitude and timescale than predicted by geochemical models. Even so, the effect of volcanism on carbonate preservation was slight compared with bolide impact. Empirical records and geochemical models support a pronounced increase in carbonate saturation state for more than 500 000 years following the mass extinction of pelagic carbonate producers at the K–Pg boundary. These examples highlight the importance of pelagic ecosystems in moderating climate and ocean chemistry.
The boron isotope-pH proxy, applied to mixed-layer planktic foraminifera, has great potential for estimating past CO2 levels, which in turn is crucial to advance our understanding of how this ...greenhouse gas influences Earth's climate. Previous culture experiments have shown that, although the boron isotopic compositions of various planktic foraminifera are pH dependent, they do not agree with the aqueous geochemical basis of the proxy. Here we outline the results of culture experiments on Globigerinoides ruber (white) across a range of pH (∼7.5–8.2) and analysed via multicollector inductively-coupled plasma mass spectrometry (MC-ICPMS), and compare these data to core-top and sediment-trap samples to derive a robust new species-specific boron isotope-pH calibration. Consistent with earlier culture studies, we show a reduced pH dependency of the boron isotopic composition of symbiont-bearing planktonic foraminifera compared to borate ion in seawater. We also present evidence for a size fraction effect in the δ11B of G. ruber. Finally, we reconstruct atmospheric CO2 concentrations over the last deglacial using our new calibration at two equatorial sites, ODP Site 999A and Site GeoB1523-1. These data provide further grounding for the application of the boron isotope-pH proxy in reconstructions of past atmospheric CO2 levels.
► We present a new δ11B-pH calibration for the planktonic foraminifera G. ruber. ► We see a lower pH sensitivity in the δ11B of G. ruber compared to δ11Bborate. ► We find a small but consequential effect of test size on G. ruber δ11B. ► We discuss the significance in terms of deciphering foraminiferal ‘vital effects’. ► Calibration applied to the last 30kyr accurately reproduces ice core values.
The response of the marine carbon cycle to changes in atmospheric CO2 concentrations will be determined, in part, by the relative response of calcifying and non-calcifying organisms to global change. ...Planktonic foraminifera are responsible for a quarter or more of global carbonate production, therefore understanding the sensitivity of calcification in these organisms to environmental change is critical. Despite this, there remains little consensus as to whether, or to what extent, chemical and physical factors affect foraminiferal calcification. To address this, we directly test the effect of multiple controls on calcification in culture experiments and core-top measurements of Globigerinoides ruber. We find that two factors, body size and the carbonate system, strongly influence calcification intensity in life, but that exposure to corrosive bottom waters can overprint this signal post mortem. Using a simple model for the addition of calcite through ontogeny, we show that variable body size between and within datasets could complicate studies that examine environmental controls on foraminiferal shell weight. In addition, we suggest that size could ultimately play a role in determining whether calcification will increase or decrease with acidification. Our models highlight that knowledge of the specific morphological and physiological mechanisms driving ontogenetic change in calcification in different species will be critical in predicting the response of foraminiferal calcification to future change in atmospheric pCO2.
In most chemical reactions, stable isotopes are fractionated in a mass-dependent manner, yielding correlated isotope ratios in elements with three or more stable isotopes. The proportionality between ...isotope ratios is set by the triple isotope fractionation exponent θ that can be determined precisely for, e.g., sulfur and oxygen by IRMS, but not for metal(loid) elements due to the lower precision of MC-ICP-MS analysis and smaller isotopic variations. Here, using Mg as a test case, we compute a complete metrologically robust uncertainty budget for apparent θ values and, with reference to this, present a new measurement approach that reduces uncertainty on θ values by 30%. This approach, namely, direct educt-product bracketing (sample–sample bracketing), allows apparent θ values of metal(loid) isotopes to be determined precisely enough to distinguish slopes in three-isotope space. For the example of Mg, we assess appropriate quality control standards for interference-to-signal ratios and report apparent θ values of carbonate–seawater pairs. We determined apparent θ values for marine biogenic carbonates, where the foraminifera Globorotalia menardii yields 0.514 ± 0.005 (2 SD), the coral Porites, 0.515 ± 0.006 (2 SD), and two specimens of the giant clam Tridacna gigas, 0.508 ± 0.007 (2 SD) and 0.509 ± 0.006 (2 SD), documenting differences in the uptake pathway of Mg among marine calcifiers. The capability to measure apparent θ values more precisely adds a new dimension to metal(loid) δ values, with the potential to allow us to resolve different modes of fractionation in industrial and natural processes.
Boron isotope ratios, as measured in planktic foraminifera, can be a useful tracer of past ocean pH, and hence help to discern the concentration of CO2 in the ancient atmosphere. However, different ...species of planktic foraminifera demonstrate different patterns of boron isotope variation with ambient seawater pH. Therefore when applying the proxy to questions in the geological past, species-specific calibrations are preferable. Beyond the evolutionary history of a calibrated species, we must rely on our understanding of the causes of the observed “vital effects” in the modern ocean, and the applicability of that understanding to extinct species. Here we present a new open-ocean calibration of the planktic foraminifera Orbulina universa, measured via Multi-Collector Inductively Coupled Mass Spectrometry (MC-ICPMS). Unlike other symbiont-bearing foraminifera, O. universa record a δ11B (and hence pH) that is lower than its surrounding seawater, but with a pH-sensitivity roughly equal to that of aqueous borate ion. We discuss the significance of this for application of the boron isotope proxy in deep time, with recommendations for best practice and future research directions.
•A new, well-constrained boron isotope calibration for O. universa.•O. universa records lower pH than ambient seawater, which we suggest is due to living at depth.•We give recommendations for accounting for vital effects in extinct species.
The clustering of mitochondria near pores in the test walls of foraminifera
suggests that these perforations play a critical role in metabolic gas
exchange. As such, pore measurements could provide a ...novel means of tracking
changes in metabolic rate in the fossil record. However, in planktonic
foraminifera, variation in average pore area, density, and porosity (the
total percentage of a test wall that is open pore space) have been variously
attributed to environmental, biological, and taxonomic drivers, complicating
such an interpretation. Here we examine the environmental, biological, and
evolutionary determinants of pore characteristics in 718 individuals,
representing 17 morphospecies of planktonic foraminifera from 6 core tops in
the North Atlantic. Using random forest models, we find that porosity is
primarily correlated to test surface area, test volume, and habitat
temperature, key factors in determining metabolic rates. In order to test if
this correlation arose spuriously through the association of cryptic species
with distinct biomes, we cultured Globigerinoides ruber in three
different temperature conditions, and found that porosity increased with
temperature. Crucially, these results show that porosity can be plastic:
changing in response to environmental drivers within the lifetime of an
individual foraminifer. This demonstrates the potential of porosity as a
proxy for foraminiferal metabolic rates, with significance for interpreting
geochemical data and the physiology of foraminifera in non-analog
environments. It also highlights the importance of phenotypic plasticity
(i.e., ecophenotypy) in accounting for some aspects of morphological
variation in the modern and fossil record.
Biweekly sediment trap samples and concurrent hydrographic measurements collected between March 2005 and October 2008 from the Cariaco Basin, Venezuela, are used to assess the relationship between ...CO32− and the area densities (ρA) of two species of planktonic foraminifera (Globigerinoides ruber (pink) and Globigerinoides sacculifer). Calcification temperatures were calculated for each sample using species‐appropriate oxygen isotope (δ18O) temperature equations that were then compared to monthly temperature profiles taken at the study site in order to determine calcification depth. Ambient CO32− was determined for these calcification depths using alkalinity, pH, temperature, salinity, and nutrient concentration measurements taken during monthly hydrographic cruises. The ρA, which is representative of calcification efficiency, is determined by dividing individual foraminiferal shell weights (±0.43 µg) by their associated silhouette areas and taking the sample average. The results of this study show a strong correlation between ρA and ambient CO32− for both G. ruber and G. sacculifer (R2 = 0.89 and 0.86, respectively), confirming that CO32− has a pronounced effect on the calcification of these species. Though the ρA for both species reveal a highly significant (p < 0.001) relationship with ambient CO32−, linear regression reveals that the extent to which CO32− influences foraminiferal calcification is species specific. Hierarchical regression analyses indicate that other environmental parameters (temperature and PO43−) do not confound the use of G. ruber and G. sacculifer ρA as a predictor for CO32−. This study suggests that G. ruber and G. sacculifer ρA can be used as reliable proxies for past surface ocean CO32−.
Key PointsForaminiferal area density & CO32‐ have a strong positive linear relationshipTemperature & PO43‐ do not significantly impact foraminiferal area densityForaminiferal area density could serve as a as reliable proxy for past CO32‐
PDF
