Posts Tagged 'paleo'

The uronic acid content of coccolith-associated polysaccharides provides insight into coccolithogenesis and past climate

Unicellular phytoplanktonic algae (coccolithophores) are among the most prolific producers of calcium carbonate on the planet, with a production of ∼1026 coccoliths per year. During their lith formation, coccolithophores mainly employ coccolith-associated polysaccharides (CAPs) for the regulation of crystal nucleation and growth. These macromolecules interact with the intracellular calcifying compartment (coccolith vesicle) through the charged carboxyl groups of their uronic acid residues. Here we report the isolation of CAPs from modern day coccolithophores and their prehistoric predecessors and we demonstrate that their uronic acid content (UAC) offers a species-specific signature. We also show that there is a correlation between the UAC of CAPs and the internal saturation state of the coccolith vesicle that, for most geologically abundant species, is inextricably linked to carbon availability. These findings suggest that the UAC of CAPs reports on the adaptation of coccolithogenesis to environmental changes and can be used for the estimation of past CO2 concentrations.

Continue reading ‘The uronic acid content of coccolith-associated polysaccharides provides insight into coccolithogenesis and past climate’

Conus Shell δ13C values as proxies for δ13CDIC in tropical waters

We use time-series δ18O and δ13C data from seawater and live-collected Conus shells from Panama’s Pacific coast to test the fidelity of the gastropod’s δ13C values as a proxy for the δ13C of marine dissolved inorganic carbon (DIC), and the potential of δ18O-δ13C correlations in shell profiles for resolving relative magnitudes of seasonal upwelling and freshening. Water samples were collected from March 2011 to August 2012 from Naos Island Marine Laboratory, and Conus specimens were collected from nearby Veracruz Beach in July 2013. In general, patterns corresponded with seasonal changes in rainfall and upwelling on the Pacific coast of Panama. During the long rainy season, the upwelling signal is absent and seawater salinity, δ18O, and δ13CDIC all decline. During the dry season, the upwelling signal increases and runoff declines increasing salinity, δ18O, and δ13CDIC values. Shell δ13C values strongly correlate with measured δ13CDIC values, but are lower than expected equilibrium for aragonite by approximately + 2‰ reflecting the incorporation of light metabolic C. The co-dependences of δ18O and δ13C provide reliable indicators of upwelling (negative correlation) and freshening (positive correlation) for nearshore environments, allowing for the study of historical climate change and upwelling based on beach-collected museum specimens.

Continue reading ‘Conus Shell δ13C values as proxies for δ13CDIC in tropical waters’

Additive effects of acidification and mineralogy on calcium isotopes in Triassic/Jurassic boundary limestones

The end-Triassic mass extinction coincided with a negative δ13C excursion, consistent with release of 13C-depleted CO2 from the Central Atlantic Magmatic Province. However, the amount of carbon released and its effects on ocean chemistry are poorly constrained. The coupled nature of the carbon and calcium cycles allows calcium isotopes to be used for constraining carbon cycle dynamics and vice versa. We present a high-resolution calcium isotope (δ44/40Ca) record from 100 m of marine limestone spanning the Triassic/Jurassic boundary in two stratigraphic sections from northern Italy. Immediately above the extinction horizon and the associated negative excursion in δ13C, δ44/40Ca decreases by ∼0.8‰ in 20 m of section and then recovers to preexcursion values. Coupled numerical models of the geological carbon and calcium cycles demonstrate that this δ44/40Ca excursion is too large to be explained by changes to seawater δ44/40Ca alone, regardless of CO2 injection volume and duration. Less than 20% of the δ44/40Ca excursion can be attributed to acidification. The remaining 80% likely reflects a higher proportion of aragonite in the original sediment, based largely on high concentrations of Sr in the samples. Our study demonstrates that coupled models of the carbon and calcium cycles have the potential to help distinguish contributions of primary seawater isotopic changes from local or diagenetic effects on the δ44/40Ca of carbonate sediments. Differentiating between these effects is critical for constraining the impact of ocean acidification during the end-Triassic mass extinction, as well as for interpreting other environmental events in the geologic past.

Continue reading ‘Additive effects of acidification and mineralogy on calcium isotopes in Triassic/Jurassic boundary limestones’

Pteropoda (Mollusca, Gastropoda, Thecosomata) from the Paleocene-Eocene Thermal Maximum (United States Atlantic Coastal Plain)

The response of many organisms to the Paleocene-Eocene Thermal Maximum (PETM; ~56 Ma) has been documented, but marine mollusks are not known from any deposits of that age. For the first time, we describe a PETM assemblage of pteropods (planktic mollusks), consisting of six species representing three genera ( Altaspiratella, Heliconoides and Limacina). Four species could be identified to species level, and one of these, Limacina novacaesarea sp. nov., is described as new. Only the genus Heliconoides was previously known from pre-Eocene sediments, with a single Campanian specimen and one latest Paleocene species. We recovered pteropods from the Marlboro Clay (United States Atlantic Coastal Plain), deposited at paleodepths from inner shelf (southern Salisbury Embayment) to middle-outer shelf (New Jersey Coastal Plain). Most living pteropod assemblages inhabit water depths of 200 m or more, so their occurrence at shelf depths may reflect transport from more open waters. During the PETM, pH in the upper waters of the ocean may have declined, but this did not cause dissolution of pteropods before they reached the seafloor, possibly due to buffering in coastal waters. The apparently sudden appearance of three genera could reflect better preservation due to high sedimentation rates, since the underlying and overlying formations show poor preservation of calcareous microfossils. Potential ancestors, however, have not been found anywhere, so we consider it more likely that the rapid environmental changes during the PETM, such as temperature, runoff and nutrient fluxes, and ocean water chemistry, may have triggered pteropod diversification.

Continue reading ‘Pteropoda (Mollusca, Gastropoda, Thecosomata) from the Paleocene-Eocene Thermal Maximum (United States Atlantic Coastal Plain)’

Comparison of Mediterranean pteropod shell biometrics and ultrastructure from historical (1910 and 1921) and present day (2012) samples provides baseline for monitoring effects of global change

Anthropogenic carbon perturbation has caused decreases in seawater pH and increases in global temperatures since the start of the 20th century. The subsequent lowering of the saturation state of CaCO3 may make the secretion of skeletons more problematic for marine calcifiers. As organisms that precipitate thin aragonite shells, thecosome pteropods have been identified as being particularly vulnerable to climate change effects. Coupled with their global distribution, this makes them ideal for use as sentinel organisms. Recent studies have highlighted shell dissolution as a potential indicator of ocean acidification; however, this metric is not applicable for monitoring pH changes in supersaturated basins. In this study, the novel approach of high resolution computed tomography (CT) scanning was used to produce quantitative 3-dimensional renderings pteropod shells to assess the potential of using this method to monitor small changes in shell biometrics that may be driven by climate change drivers. An ontogenetic analysis of the shells of Cavolinia inflexa and Styliola subula collected from the Mediterranean was used to identify suitable monitoring metrics. Modern samples were then compared to historical samples of the same species, collected during the Mediterranean leg of the Thor (1910) and Dana (1921) cruises to assess whether any empirical differences could be detected. Shell densities were calculated and scanning electron microscopy was used to compare the aragonite crystal morphology. pH for the collection years was hind-cast using temperature and salinity time series with atmospheric CO2 concentrations from ice core data. Historical samples of S. subula were thicker than S. subula shells of the same size from 2012 and C. inflexa shells collected in 1910 were significantly denser than those from 2012. These results provide a baseline for future work to develop monitoring techniques for climate change in the oceans using the novel approach of high-resolution CT scanning.

Continue reading ‘Comparison of Mediterranean pteropod shell biometrics and ultrastructure from historical (1910 and 1921) and present day (2012) samples provides baseline for monitoring effects of global change’

Episode of intense chemical weathering during the termination of the 635 Ma Marinoan glaciation

Cryogenian (∼720–635 Ma) global glaciations (the snowball Earth) represent the most extreme ice ages in Earth’s history. The termination of these snowball Earth glaciations is marked by the global precipitation of cap carbonates, which are interpreted to have been driven by intense chemical weathering on continents. However, direct geochemical evidence for the intense chemical weathering in the aftermath of snowball glaciations is lacking. Here, we report Mg isotopic data from the terminal Cryogenian or Marinoan-age Nantuo Formation and the overlying cap carbonate of the basal Doushantuo Formation in South China. A positive excursion of extremely high δ26Mg values (+0.56 to +0.95)—indicative of an episode of intense chemical weathering—occurs in the top Nantuo Formation, whereas the siliciclastic component of the overlying Doushantuo cap carbonate has significantly lower δ26Mg values (<+0.40), suggesting moderate to low intensity of chemical weathering during cap carbonate deposition. These observations suggest that cap carbonate deposition postdates the climax of chemical weathering, probably because of the suppression of carbonate precipitation in an acidified ocean when atmospheric CO2 concentration was high. Cap carbonate deposition did not occur until chemical weathering had consumed substantial amounts of atmospheric CO2 and accumulated high levels of oceanic alkalinity. Our finding confirms intense chemical weathering at the onset of deglaciation but indicates that the maximum weathering predated cap carbonate deposition.

Continue reading ‘Episode of intense chemical weathering during the termination of the 635 Ma Marinoan glaciation’

Early to Late Maastrichtian environmental changes in the Indian Ocean compared with Tethys and South Atlantic

Planktic foraminiferal analysis, including species populations, diversity trends, high-stress indices and stable isotopes of the latest Campanian through Maastrichtian in the South Atlantic, Tethys and Indian oceans reveal four major climate and faunal events that ended with the Cretaceous-Paleogene (K/Pg), formerly Cretaceous-Tertiary (K/T), mass extinction. The prelude to these events is the late Campanian cooling that reached minimum temperatures in the earliest Maastrichtian (base C31r) correlative with low primary productivity and species diversity. Event-1 begins during the persistent cool climate of the early Maastrichtian (lower C31r) when primary productivity rapidly increased accompanied by rapid species originations, attributed to increased nutrient influx from increased upwelling, erosion during the sea-level fall ~ 70.6 Ma, and Ninety East Ridge volcanism. During Event-2 (upper C31r to lower C30n), climate rapidly warmed by 2–3 °C in deep waters and peaked at 22 °C on land, primary productivity remained high and diversification reached maximum for the entire Cretaceous. We attribute this climate warming to intense Ninety East Ridge volcanic activity beginning ~ 69.5 Ma, accompanied by rapid reorganization of intermediate oceanic circulation. Enhanced greenhouse conditions due to the eruption of Deccan Phase-1 in India resulted in detrimental conditions for planktic foraminifera marking the end of diversification. Global cooling resumed in Event-3 (C30n), species diversity declined gradually accompanied by dwarfing, decreased large specialized species, increased small ecologically tolerant taxa, and ocean acidification. Event-3 is mainly the result of enhanced weathering and volcanogenic CO2 adsorption by the oceans during the preceding warm Event-2 that led to cooling and lower pH in the surface ocean. Event-4 marks the last 250 kyr of the Maastrichtian (C29r), which began with the largest Deccan eruptions (Phase-2) that caused rapid climate warming of 4 °C in deep waters and 8 °C on land, acid rain and ocean acidification leading to a major carbonate crisis preceding the K/T mass extinction.

Continue reading ‘Early to Late Maastrichtian environmental changes in the Indian Ocean compared with Tethys and South Atlantic’

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