Posts Tagged 'paleo'

Very large release of mostly volcanic carbon during the Palaeocene–Eocene Thermal Maximum

The Palaeocene–Eocene Thermal Maximum1, 2 (PETM) was a global warming event that occurred about 56 million years ago, and is commonly thought to have been driven primarily by the destabilization of carbon from surface sedimentary reservoirs such as methane hydrates3. However, it remains controversial whether such reservoirs were indeed the source of the carbon that drove the warming1, 3, 4, 5. Resolving this issue is key to understanding the proximal cause of the warming, and to quantifying the roles of triggers versus feedbacks. Here we present boron isotope data—a proxy for seawater pH—that show that the ocean surface pH was persistently low during the PETM. We combine our pH data with a paired carbon isotope record in an Earth system model in order to reconstruct the unfolding carbon-cycle dynamics during the event6, 7. We find strong evidence for a much larger (more than 10,000 petagrams)—and, on average, isotopically heavier—carbon source than considered previously8, 9. This leads us to identify volcanism associated with the North Atlantic Igneous Province10, 11, rather than carbon from a surface reservoir, as the main driver of the PETM. This finding implies that climate-driven amplification of organic carbon feedbacks probably played only a minor part in driving the event. However, we find that enhanced burial of organic matter seems to have been important in eventually sequestering the released carbon and accelerating the recovery of the Earth system

Continue reading ‘Very large release of mostly volcanic carbon during the Palaeocene–Eocene Thermal Maximum’

Influence of solution chemistry on the boron content in inorganic calcite grown in artificial seawater

The ratio of boron to calcium (B/Ca) in marine biogenic carbonates has been proposed as a proxy for properties of seawater carbonate chemistry. Applying this proxy to planktic foraminifera residing in the surface seawater largely in equilibrium with the atmosphere may provide a critical constraint on past atmospheric CO2 concentrations. However, precise controls on B/Ca in planktic foraminifera remain enigmatic because it has been shown to depend on multiple physicochemical seawater properties. To help establish a firm inorganic basis for interpreting the B/Ca records, we examined the effect of a suite of chemical parameters ([Ca2+], pH, [DIC], salinity and [PO43-]) on B/Ca in inorganic calcite precipitated in artificial seawater. These parameters were primarily varied individually while keeping all others constant, but we also tested the influence of pH and [DIC] at a constant calcite precipitation rate (R) by concurrent [Ca2+] adjustments. In the simple [Ca2+], pH and [DIC] experiments, both R and B/Ca increased with these parameters. In the pH–[Ca2+] and [DIC]–[Ca2+] experiments at constant R, on the other hand, B/Ca was invariant at different pH and decreased with [DIC], respectively. These patterns agree with the behavior of solution [BTotal/DIC] ratio such that, at a fixed [BTotal], it is independent of pH but decreases with [DIC]. Based on these results, R and [BTotal/DIC] ratio appear to be the primary controls on B/Ca in inorganic calcite, suggesting that both B(OH)4 and B(OH)3 are possibly involved in B incorporation. Moreover, B/Ca modestly increased with salinity and [PO43-]. Inorganic calcite precipitated at higher R and in the presence of oxyanions such as SO42- and PO43- in growth solutions often undergoes surface roughening due to formation of crystallographic defects, vacancies and, occasionally, amorphous/hydrous CaCO3.These non-lattice sites may provide additional space for B, particularly B(OH)3. Consequently, besides the macroscopic influence of R and bulk solution chemistry, molecular-scale processes associated with calcite nucleation can be an important consideration for B incorporation, especially in complex ionic solutions. Lastly, the covariance of B/Ca with [DIC] and salinity observed here qualitatively agrees with those in planktic foraminifers. It follows that their impact on foraminiferal B/Ca is partly inorganically driven, which may explain why the effect is evident across different species.

Continue reading ‘Influence of solution chemistry on the boron content in inorganic calcite grown in artificial seawater’

Modern planktic foraminifers in the high-latitude ocean

Highlights

  • We review the knowledge on modern high-latitude planktic foraminifers.
  • Subpolar species currently invade higher latitudes.
  • Climate change affects phenology, seawater pH, and carbon turnover.
  • Modern planktic foraminifers are briefly discussed for their paleoceanographic significance.


Abstract

Planktic foraminifers can be sensitive indicators of the changing environment including both the Arctic Ocean and Southern Ocean. Due to variability in their ecology, biology, test characteristics, and fossil preservation in marine sediments, they serve as valuable archives in paleoceanography and climate geochemistry over the geologic time scale. Foraminifers are sensitive to, and can therefore provide proxy data on ambient water temperature, salinity, carbonate chemistry, and trophic conditions through shifts in assemblage (species) composition and the shell chemistry of individual specimens. Production and dissolution of the calcareous shell, as well as growth and remineralization of the cytoplasm, affect the carbonate counter pump and to a lesser extent the soft-tissue pump, at varying regional and temporal scales. Diversity of planktic foraminifers in polar waters is low in comparison to lower latitudes and is limited to three native species: Neogloboquadrina pachyderma, Turborotalita quinqueloba, and Globigerina bulloides, of which N. pachyderma is best adapted to polar conditions in the surface ocean. Neogloboquadrina pachyderma hibernates in brine channels in the lower layers of the Antarctic sea ice, a strategy that is presently undescribed in the Arctic. In open Antarctic and Arctic surface waters T. quinqueloba and G. bulloides increase in abundance at lower polar to subpolar latitudes and Globigerinita uvula, Turborotalita humilis, Globigerinita glutinata, Globorotalia inflata, and Globorotalia crassaformis complement the assemblages. Over the past two to three decades there has been a marked increase in the abundance of Orcadia riedeli and G. uvula in the subpolar and polar Indian Ocean, as well as in the northern North Atlantic. This paper presents a review of the knowledge of polar and subpolar planktic foraminifers. Particular emphasis is placed on the response of foraminifers to modern warming and ocean acidification at high latitudes and the implications for data interpretation in paleoceanography and paleoclimate research.

Continue reading ‘Modern planktic foraminifers in the high-latitude ocean’

Retrodiction of secular variations in deep-sea CaCO3 burial during the Cenozoic

Deep-sea sediments record changes in oceanic carbonate chemistry and CaCO3sedimentation rate through temporal variations in the total burial of CaCO3 and the position of the carbonate snowline, i.e., the ocean depth at which CaCO3-free sediments are first recorded. This paper links mathematically secular changes in snowline to those in the burial rate through a set of relatively simple equations. When the available Cenozoic deep-sea burial records are employed to predict secular variations in snowline, the process fails at some time in the past, indicating that these records are not consistent with each other. The burial records are more likely the source of this problem, as they involve far more uncertainties than the snowline records. As a consequence, we introduce a method for estimating carbonate burial through the use of a canonical CaCO3-depth profile, which can respond dynamically to secular changes in carbonate sedimentation and the positions of both the snowline and the carbonate saturation horizon. The resulting synthetic CaCO3burial record is consistent with snowline records and indicates that the burial rates offered by Davies and Worsley (1981) are generally too high, with highly questionable maxima at 25 and 47 Ma BP. Our estimates of burial are more consistent with the range advanced by Mackenzie and Morse (1992); nevertheless, our results differ from the latter with respect to timing and magnitude of the variations. Our approach allows simultaneous calculation of the mean carbonate ion concentration of the deep sea. We find that carbonate-ion levels fell through the Cenozoic and are similar to those calculated by Tyrrell and Zeebe (2004), using a different model. Secular variations in CaCO3 burial are found to be primarily driven by changes in the Ca2+–CO32 ion product within the bottom-waters, with an increase in the sedimentation rate of CaCO3 of secondary importance over the Cenozoic.

Continue reading ‘Retrodiction of secular variations in deep-sea CaCO3 burial during the Cenozoic’

Climate, anchovy, and sardine

Anchovy and sardine populated productive ocean regions over hundreds of thousands of years under a naturally varying climate, and are now subject to climate change of equal or greater magnitude occurring over decades to centuries. We hypothesize that anchovy and sardine populations are limited in size by the supply of nitrogen from outside their habitats originating from upwelling, mixing, and rivers. Projections of the responses of anchovy and sardine to climate change rely on a range of model types and consideration of the effects of climate on lower trophic levels, the effects of fishing on higher trophic levels, and the traits of these two types of fish. Distribution, phenology, nutrient supply, plankton composition and production, habitat compression, fishing, and acclimation and adaptation may be affected by ocean warming, acidification, deoxygenation, and altered hydrology. Observations of populations and evaluation of model skill are essential to resolve the effects of climate change on these fish.

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The survival, recovery, and diversification of metazoan reef ecosystems following the end-Permian mass extinction event

The Triassic Period records important ecological transitions in the aftermath of the end-Permian mass extinction and is a key interval in the evolution of modern coral reefs. There have been several critical developments in our understanding of Triassic reef evolution over the past decade: the timing of events and duration of stages have changed dramatically; the discovery of metazoan reefs in the Early Triassic; details about the environmental perturbations that drove the extinction; the relationship between tectonic activity and platform margin reef proliferation; and additional proxy evidence for the co-evolution of coral reef-builders and their photosymbionts. Here, we provide an up-to-date synthesis of reef collapse and recovery dynamics following the end-Permian extinction, specifically integrating recent discoveries. The evolution of reef ecosystems can be divided into five phases based on their composition. 1) Microbial-metazoan reefs represent survival communities that characterize the immediate extinction aftermath. 2) The re-establishment of reefs built by metazoans (small sponge biostromes and bivalve buildups) is observed in oxygenated settings in the Olenekian (Early Triassic). 3) Towards the end of the Olenekian and into the Anisian (Middle Triassic) low-diversity, “Tubiphytes”-dominated reefs formed, which represent the first Triassic platform-margin reefs; platform-margin reefs, however, are not widespread until the late Anisian. 4) Late Anisian reefs also record a composition change and increase in species richness with sponges and “Tubiphytes” as the main reef builders. 5) The first scleractinian corals (which are the main reef builder in modern marine reef ecosystems) evolved during the Anisian but are not reported as dominant reef builders until the Late Triassic. The radiation of coral reefs is posited to be coupled to the acquisition of photosymbionts (e.g., zooxanthellae). There is clearly a stepwise evolution of reef types during the Triassic; however, once each reef type appears it persists throughout the remainder of the Triassic. The survival, recovery, and diversification of reef ecosystems is, therefore, more complex than previously outlined, particularly with respect to the earliest post-extinction ecosystems. These recent advances highlight the need to thoroughly document the faunal compositions of understudied reef systems as well as to continue the exploration of Triassic ecosystems in underrepresented regions.

Continue reading ‘The survival, recovery, and diversification of metazoan reef ecosystems following the end-Permian mass extinction event’

Calibration of the carbon isotope composition (δ13C) of benthic foraminifera

The carbon isotope composition (δ13C) of seawater provides valuable insight on ocean circulation, air-sea exchange, the biological pump, and the global carbon cycle and is reflected by the δ13C of foraminifera tests. Here more than 1700 δ13C observations of the benthic foraminifera genus Cibicides from late Holocene sediments (δ13CCibnat) are compiled and compared with newly updated estimates of the natural (preindustrial) water column δ13C of dissolved inorganic carbon (δ13CDICnat) as part of the international Ocean Circulation and Carbon Cycling (OC3) project. Using selection criteria based on the spatial distance between samples, we find high correlation between δ13CCibnat and δ13CDICnat, confirming earlier work. Regression analyses indicate significant carbonate ion (−2.6 ± 0.4) × 10−3‰/(μmol kg−1) [CO32−] and pressure (−4.9 ± 1.7) × 10−5‰ m−1 (depth) effects, which we use to propose a new global calibration for predicting δ13CDICnat from δ13CCibnat. This calibration is shown to remove some systematic regional biases and decrease errors compared with the one-to-one relationship (δ13CDICnat = δ13CCibnat). However, these effects and the error reductions are relatively small, which suggests that most conclusions from previous studies using a one-to-one relationship remain robust. The remaining standard error of the regression is generally σ ≅ 0.25‰, with larger values found in the southeast Atlantic and Antarctic (σ ≅ 0.4‰) and for species other than Cibicides wuellerstorfi. Discussion of species effects and possible sources of the remaining errors may aid future attempts to improve the use of the benthic δ13C record.

Continue reading ‘Calibration of the carbon isotope composition (δ13C) of benthic foraminifera’


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