Posts Tagged 'paleo'

An assessment of reef coral calcification over the late Cenozoic

Shallow-water reef-building corals have an extensive geological record and many aspects of their evolution, biodiversity, and biogeography are known in great details. In contrast, the adaptive potential and risk of extinction of coral reefs in response to excessive warming and ocean acidification remains largely undocumented. It is well established that anthropogenic CO2 emissions cause global warming and ocean acidification (lowering of pH), which increasingly impede the biomineralization process in many marine calcareous biota. The “light-enhanced” calcification machinery of the shallow-water reef corals is particularly threatened by this development through the combined effect of a lowering of the supersaturation of seawater with CaCO3 (aragonite) and an expulsion of the symbiotic zooxanthellae (bleaching). The bleaching is of prime importance, because it interrupts the supply of DIC and metabolites required for pH upregulation within the calcification fluid. The degree of calcification in scleractinian reef corals may therefore represent a suitable tracer to assess the state of the ocean carbonate system and the photosynthetic performance of the zooxanthellae during past episodes of natural environmental change. This study presents the first comprehensive set of calcification data from corals covering the early Miocene to early Pleistocene interval (20.8 to 1.2 million years, Ma). Various screening procedures ensured that the studied coral skeletons are pristine and suited to yield meaningful stable isotope data (δ18O, δ13C) and calcification records. δ18O and δ13C values document growth environments consistent with current tropical and subtropical settings. To assess fossil calcification rates, we use a reference dataset of recent corals from the Indo-Pacific (Porites) and an independent validation dataset from the Western Atlantic-Caribbean (Orbicella). Almost all fossil corals document very low annual rates of upward growth (“extension rate”) relative to present, and lower skeletal bulk density than predicted by established modern relationships. To allow for a quantitative assessment of coral calcification performance, we use a new approach that we term the calcification anomaly. It is insensitive to sea-surface temperature and well-suited for comparative assessments of calcification performance between reef sites and over time. Based on this approach, the majority of fossil corals in our dataset displays hypo-calcification, while a few show optimal calcification and none display hyper-calcification. Compared to present-day growth conditions, the fossil calcification data show that (1) skeletogenesis responded in a fully compatible way to known environmental stresses (e.g. turbid water, elevated salinity, eutrophy), and that (2) the calcification performance within the reef window (i.e. oligotrophic clear-water settings) remained below that of modern z-corals. Since fossil coral δ13C values are compatible with those of modern reef corals, we infer that the light-enhanced calcification system of symbiotic scleractinian corals was fully established by the beginning of the Neogene and that lower-than-present calcification performance was the likely response to a chronically low pH and/or low carbonate saturation state of the global ocean. If so, the present-day saturation state appears to be rather an exception than the norm and probably not a suitable starting point for predicting future calcification trends. In addition, using trends from the geological past does not include anthropogenic side-effects such as eutrophication and pollution.

Continue reading ‘An assessment of reef coral calcification over the late Cenozoic’

Marine clade sensitivities to climate change conform across timescales

Rapid climate change is postulated to cause marine extinctions, especially among climate-sensitive clades, traits and regions1,2,3,4,5,6. This premise is based on two hypotheses: (1) known individual physiological sensitivities scale up to macroecological selectivity patterns4,7,8 and (2) ancient hyperthermal events are appropriate models to anticipate ecological winners and losers of anthropogenic climate change9. Yet these hypotheses have largely escaped quantitative appraisal. Here we show that experimental responses of modern marine ectotherms to single and combined climate-related stressors (such as seawater warming, hypoxia and acidification) align with Phanerozoic fossil extinction regimes across clades and functional traits. Of climate-related stressors, the synergistic interaction between warming and hypoxia10, encumbering aerobic metabolism, has the greatest potency as a proximate driver of extinction. All else being equal8, this synergy particularly imperils modern warm-water organisms. Modern–fossil agreement is strongest at intermediate–high extinction intensities and hyperthermal events but may fail at extreme extinction events, perhaps due to rising prominences of, and interactions among, additional biotic and abiotic stressors. According to results from marine ectotherms, clade-based sensitivity of individuals to climate-related stressors scales up from subannual experiments and decadal range-shift response magnitudes11, to extinction selectivity patterns at ancient climate-related stressor events and the Phanerozoic durations of genera.

Continue reading ‘Marine clade sensitivities to climate change conform across timescales’

Aragonite pteropod abundance and preservation records from the Maldives, equatorial Indian Ocean: inferences on past oceanic carbonate saturation and dissolution events

Highlights

• 1.2 Myr record of pteropod abundance/preservation variations from the Maldives

• Periods of enhanced ventilation during MIS 8, 3, 2 and MIS 14-13, 6-5 transitions

• MBDI marked by very poor preservation of pteropods during MIS 13 to 11

• Seawater carbonate chemistry plays a role in shell calcification.

• Glacial periods, MIS 16, 14, 6, 4, 2 are marked by larger and pristine shells.

Abstract

During the International Ocean Discovery Program (IODP) Expedition 359, a long continuous carbonate-rich sequence was recovered from the Inner Sea of Maldives. We investigated pteropod proxies (absolute abundance of pteropods species, total pteropods, epipelagic to mesopelagic ratio, fragmentation ratio, Limacina Dissolution Index (LDX), mean shell size variations of L. inflata) from Sites U1467 (water depth: 487 m) and U1468 (water depth: 521 m) to understand both surface and sub-surface paleoceanographic changes in the equatorial Indian Ocean and to improve our understanding of the factors responsible for pteropod preservation on longer timescales. A total of 15 species of pteropods were identified, and their downcore variations were documented from the core top to 707.49 mbsf in U1467 and from 447.4 to 846.92 mbsf in U1468. At the Site U1467, pteropod shells show high abundances/preservation up to a depth of 45 mbsf (~1.2 Ma), which is consistent with the presence of aragonite content in sediments (with the top 50 m bearing high aragonite content). Beyond 45 mbsf, only fragmented pteropod shells were seen down to 50 mbsf (corresponding to 1.5 Ma) followed by a total absence of pteropod shells and fragments from 50 mbsf (~1.5 Ma) to the end of the core at 846.92 mbsf (~24 Ma). A decrease in the SO42ˉconcentration and alkalinity in the interstitial fluid geochemistry is seen at these depths. The presence of dolomite content below 50 mbsf also indicates the alteration of aragonite into dolomite. Analyses of the carbonate preservation proxies reveal that the pteropods exhibit considerable fluctuation in abundance/preservation during the last 1.2 Myr. A good to moderate preservation (LDX: 2 to 3) is seen which correlates well with the fragmentation ratio but with an inverse relation with calcification rate. The proxies for in-life pteropod shell dissolution (average size of L. inflata and LDX) indicate that glacial periods (MIS 16, 14, 6, 4 and 2) have shown no signs of dissolution pointing better calcification under aragonite-saturated water column which is in good correlation with reduced atmospheric CO₂ concentration. Epipelagic/mesopelagic ratio indicates that the water column exhibited enhanced ventilation and mixing during glacial to interglacial periods, but intervals of intense stratification, a sign of poor ventilation or weakened circulation, was prevalent beyond MIS 14. The longest interval of poorest preservation was marked during MIS 11 and 13, which corresponds to the ‘Mid-Brunhes Dissolution Interval (MBDI).’ On a longer time scale, the abundances/preservation of pteropods in the Maldives seems to be controlled by changes in the seawater chemistry associated with monsoon productivity, water column ventilation, and atmospheric CO2 concentration.

Continue reading ‘Aragonite pteropod abundance and preservation records from the Maldives, equatorial Indian Ocean: inferences on past oceanic carbonate saturation and dissolution events’

Establishing the link between Permian volcanism and biodiversity changes: insights from geochemical proxies

Highlights

• Current understanding of biodiversity changes in the Permian is summarized.

• Conventional and non-traditional geochemical proxy records in the Permian are assessed.

• Main characteristics of four Permian large igneous provinces are compared.

• The potential links between the Siberian Traps and EPME, and the Emeishan LIP and EGME, are examined.

• In addition to the Siberian Traps, continental arc magmatism could also played an important role in the EPME.

Abstract

Current understanding of biodiversity changes in the Permian is presented, especially the consensus and disagreement on the tempo, duration, and pattern of end-Guadalupian and end-Permian mass extinctions. The end-Guadalupian mass extinction (EGME; i.e., pre-Lopingian crisis) is not as severe as previously thought. Moreover, the turnovers of major fossil groups occurred at different temporal levels, therefore the total duration of the end-Guadalupian mass extinction is relatively extended. By comparison, fossil records constrained with high-precision geochronology indicate that the end-Permian mass extinction (EPME) was a single-pulse event and happened geologically instantaneous. Variation of geochemical proxies preserved in the sedimentary records is important evidence in examining potential links between volcanisms and biodiversity changes. Some conventional and non-traditional geochemical proxy records in the Permian show abrupt changes across the Permian-Triassic boundary, reflecting climate change, ocean acidification and anoxia, carbon cycle perturbation, gaseous metal loading, and enhanced continental weathering. These, together with the stratigraphic coincidence between volcanic ashes and the end-Permian mass extinction horizon, point to large-scale volcanism as a potential trigger mechanism.

To further define the nature of volcanism which was responsible for global change in biodiversity, main characteristics of four Permian large igneous provinces (LIPs; i.e., Tarim, Panjal, Emeishan, and Siberian) are compared, in terms of timing and tempo, spatial distribution and volume, and magma-wall rock interactions. The comparison indicates that volcanic fluxes (i.e., eruption rates) and gas productions are the key features distinguishing the Siberian Traps from other LIPs, which also are the primary factors in determining the LIP’s potential of affecting Earth’s surface system. We find that the Siberian Traps volcanism, especially the switch from dominantly extrusive eruptions to widespread sill intrusions, has the strongest potential for destructive impacts, and most likely is the ultimate trigger for profound environmental and biological changes in the latest Permian-earliest Triassic. The role of Palaeotethys subduction-related arc magmatism cannot be fully ruled out, given its temporal coincidence with the end-Permian mass extinction. As for the Emeishan LIP, medium volcanic flux and gas emission probably limited its killing potential, as evident from weak changes in geochemical proxies and biodiversity. Because of its long-lasting but episodic nature, the Early Permian magmatism (e.g., Tarim, and Panjal) may have played a positive role in affecting the contemporaneous environment, as implicated by coeval progressive climate warming, termination of the Late Palaeozoic Ice Age (LPIA), and flourishing of ecosystems.

Continue reading ‘Establishing the link between Permian volcanism and biodiversity changes: insights from geochemical proxies’

The magnitude of surface ocean acidification and carbon release during Eocene Thermal Maximum 2 (ETM‐2) and the Paleocene–Eocene Thermal Maximum (PETM)

Eocene Thermal Maximum 2 (ETM‐2; 54.1 Ma) was the second largest Eocene hyperthermal. Like the Paleocene–Eocene Thermal Maximum (PETM), ETM‐2 was characterized by massive carbon emissions and several degrees of global warming, thus can serve as a case study for assessing the impacts of rapid CO2 emissions on ocean carbonate chemistry, biota and climate. Marine carbonate records of ETM‐2 are better preserved than those of the PETM due to more subdued carbonate dissolution. As yet, however, the magnitude of this carbon cycle perturbation has not been well constrained. Here, we present the first records of surface ocean acidification for ETM‐2, based on stable boron isotope records in mixed‐layer planktic foraminifera from two mid‐latitude ODP Sites (1210 in the N. Pacific and 1265 in the S.E. Atlantic), which indicate conservative minimum global sea surface acidification of –0.20 +0.12/–0.13 pH units. Using these estimates of pH and temperature as constraints on carbon cycle model simulations, we conclude that the total mass of C, released over a period of 15 to 25 kyr during ETM‐2, likely ranged from 2,600 to 3,800 Gt C, which is greater than previously estimated on the basis of other observations (i.e., stable carbon isotopes and carbonate compensation depth) alone.

Continue reading ‘The magnitude of surface ocean acidification and carbon release during Eocene Thermal Maximum 2 (ETM‐2) and the Paleocene–Eocene Thermal Maximum (PETM)’

Reconstructing 800 years of carbonate ion concentration in the Cariaco basin using the area density of planktonic foraminifera shells

Anthropogenically mediated ocean acidification (OA) has negative impacts on many marine organisms, especially calcifiers. However, systematic measurements of OA have only been made over the past four decades. In order to improve future predictions and understand how ongoing OA compares to natural variability on longer timescales, it is critical to extend records beyond observational time series. In the Cariaco Basin, located in the tropical Atlantic, near‐surface dissolved inorganic carbon reflects atmospheric carbon dioxide concentrations (CO2) since the Industrial Revolution, making it an ideal site for examining longer‐term variability. We extend the record of Cariaco Basin near‐surface [CO32−] back to 1240 CE, using the area density (shell weight (μg)/shell area (μm2)) of the planktonic foraminifer Globigerinoides ruber (pink). Multidecadal variability is observed throughout the record. Since the Industrial Revolution (1760–2007 CE), [CO32−] has declined by 0.22 μmol kg−1 year−1, in agreement with the magnitude and direction of change captured in the shorter instrumental time series. During the Little Ice Age (1500–1760 CE), a period marked by regional drought, substantial variability but no long‐term trend is observed, while a decrease in [CO32−] of 0.11 μmol kg−1 y−1 occurs at the end of the Medieval Climate Anomaly (MCA) (1240 – 1500 CE). Both the MCA and Little Ice Age contain substantial natural variability in near surface [CO32−] that we attribute to changes in regional upwelling and atmospheric CO2. However, the decline in [CO32−] occurring in the Post‐Industrial Period is anomalous against a backdrop of 800 years of natural variability, reflecting OA associated with anthropogenic increases in atmospheric CO2.

Continue reading ‘Reconstructing 800 years of carbonate ion concentration in the Cariaco basin using the area density of planktonic foraminifera shells’

Calcium isotope evidence for environmental variability before and across the Cretaceous-Paleogene mass extinction

Carbon dioxide release during Deccan Traps volcanism and the Chicxulub impact likely contributed to the Cretaceous-Paleogene (K-Pg) mass extinction; however, the intensity and duration of CO2 input differed between the two events. Large and rapid addition of CO2 to seawater causes transient decreases in pH, [CO32–], and carbonate mineral saturation states. Compensating mechanisms, such as dissolution of seafloor sediment, reduced biomineralization, and silicate weathering, mitigate these effects by increasing the same parameters. The calcium isotope ratios (δ44/40Ca) of seawater and marine carbonates are hypothesized to respond to these perturbations through weathering/carbonate deposition flux imbalances and/or changes in fractionation between carbonate minerals and seawater. We used a high-precision thermal ionization mass spectrometry method to measure δ44/40Ca values of aragonitic bivalve and gastropod mollusk shells from the K-Pg interval of the López de Bertodano Formation on Seymour Island, Antarctica. Well-preserved shells spanning the late Maastrichtian (ca. 67 Ma) to early Danian (ca. 65.5 Ma) have δ44/40Ca values ranging from −1.89‰ to −1.57‰ (seawater [sw]). Shifts in δ44/40Ca inversely correlate with sedimentological indicators of saturation state. A negative excursion begins before and continues across the K-Pg boundary. According to a simple mass-balance model, neither input/output flux imbalances nor change in the globally integrated bulk carbonate fractionation factor can produce variations in seawater δ44/40Ca sufficient to explain the measured trends. The data are consistent with a dynamic molluscan Ca isotope fractionation factor sensitive to the carbonate geochemistry of seawater. The K-Pg extinction appears to have occurred during a period of carbonate saturation state variability caused by Deccan volcanism.

Continue reading ‘Calcium isotope evidence for environmental variability before and across the Cretaceous-Paleogene mass extinction’


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