Posts Tagged 'paleo'

The palaeoceanographic crisis of the Early Aptian (OAE 1a) in the Vocontian Basin (SE France)

Highlights

• Palaeoceanographic changes across the OAE 1a investigated in the Vocontian Basin.
• Cyclostratigraphic analysis give for the OAE 1a a duration of 1.2 Myr.
• Onset of both carbonate platform and nannoconid crises not linked to the OAE 1a.
• Shallowing of the calcite CCD suspected in the lower part of the OAE 1a.
• Deep water anoxia and enhanced fertility in the upper part of the OAE 1a.

Abstract

The Vocontian Basin (SE France) which presents lower Aptian expanded successions characterized by major lithological changes, is particularly suitable to determine palaeoenvironmental changes occurring across the OAE 1a. A multidisciplinary study (sedimentology, CaCO3, TOC, carbon and oxygen stable isotopes, micropalaeontology, cyclostratigraphy) was carried out in the Notre-Dame-de-Rosans section in order to establish a detailed chronological framework of these changes. The OAE 1a corresponds to carbon isotope segments C3–C6, and it lasted for 1.2 Myr. A first drop in the carbonate production occurs 500 kyr before the OAE 1a (upper part of the carbon isotope segment C2), and could result from both onset of “Urgonian” carbonate platform demise and associated reducing export of platform-derived sediments into the basin, and nannoconid crisis. A second drastic drop (crash) in the carbonate production is recorded within the lower part of the OAE 1a (latter part of segment C3 to C5) and is explained by a strong dissolution. This study then shows that the onset of the major carbonate crisis, that occurs before the OAE 1a, could be due to both rise in sea-level and in marine surface water fertility, whereas its “acme” that occurs within the OAE 1a, could be related to CO2-induced ocean acidification. Black shales of the “Niveau Goguel” occur in the upper part of the OAE 1a and represent the lower part of segment C6. Surface-waters primary producers are principally represented by cyanobacteria, whereas nannofossil primary productivity is reduced, and deep-water anoxia prevailed during the deposition of “the Niveau Goguel”. The last 300 kyr of the OAE 1a are characterized by a partial recovery of both nannofossil primary productivity and pelagic carbonate production, which sharply increase just after the end of the event. This study also shows that organic-rich layers associated to the OAE 1a are diachronous in the Tethyan realm.

Continue reading ‘The palaeoceanographic crisis of the Early Aptian (OAE 1a) in the Vocontian Basin (SE France)’

Little lasting impact of the Paleocene-Eocene Thermal Maximum on shallow marine molluscan faunas

Global warming, acidification, and oxygen stress at the Paleocene-Eocene Thermal Maximum (PETM) are associated with severe extinction in the deep sea and major biogeographic and ecologic changes in planktonic and terrestrial ecosystems, yet impacts on shallow marine macrofaunas are obscured by the incompleteness of shelf sections. We analyze mollusk assemblages bracketing (but not including) the PETM and find few notable lasting impacts on diversity, turnover, functional ecology, body size, or life history of important clades. Infaunal and chemosymbiotic taxa become more common, and body size and abundance drop in one clade, consistent with hypoxia-driven selection, but within-clade changes are not generalizable across taxa. While an unrecorded transient response is still possible, the long-term evolutionary impact is minimal. Adaptation to already-warm conditions and slow release of CO2 relative to the time scale of ocean mixing likely buffered the impact of PETM climate change on shelf faunas.

Continue reading ‘Little lasting impact of the Paleocene-Eocene Thermal Maximum on shallow marine molluscan faunas’

Strategies in times of crisis—insights into the benthic foraminiferal record of the Palaeocene–Eocene Thermal Maximum

Climate change is predicted to alter temperature, carbonate chemistry and oxygen availability in the oceans, which will affect individuals, populations and ecosystems. We use the fossil record of benthic foraminifers to assess developmental impacts in response to environmental changes during the Palaeocene–Eocene Thermal Maximum (PETM). Using an unprecedented number of µ-computed tomography scans, we determine the size of the proloculus (first chamber), the number of chambers and the final size of two benthic foraminiferal species which survived the extinction at sites 690 (Atlantic sector, Southern Ocean, palaeodepth 1900 m), 1210 (central equatorial Pacific, palaeodepth 2100 m) and 1135 (Indian Ocean sector, Southern Ocean, palaeodepth 600–1000 m). The population at the shallowest site, 1135, does not show a clear response to the PETM, whereas those at the other sites record reductions in diameter or proloculus size. Temperature was similar at all sites, thus it is not likely to be the reason for differences between sites. At site 1210, small size coincided with higher chamber numbers during the peak event, and may have been caused by a combination of low carbonate ion concentrations and low food supply. Dwarfing at site 690 occurred at lower chamber numbers, and may have been caused by decreasing carbonate saturation at sufficient food levels to reproduce. Proloculus size varied strongly between sites and through time, suggesting a large influence of environment on both microspheric and megalospheric forms without clear bimodality. The effect of the environmental changes during the PETM was more pronounced at deeper sites, possibly implicating carbonate saturation.

Continue reading ‘Strategies in times of crisis—insights into the benthic foraminiferal record of the Palaeocene–Eocene Thermal Maximum’

Placing our current ‘hyperthermal’ in the context of rapid climate change in our geological past

It is widely recognized that anthropogenic climate change and ocean acidification resulting from the emission of vast quantities of CO2 and other greenhouse gases pose a considerable threat to ecosystems and modern society. Global temperatures are already warmer today than at any time in at least the last 2000 years [1], and unabated use of fossil fuel will cause continued warming and sea-level rise potentially for millennia as the climate system slowly adjusts to the enhanced greenhouse effect [2]. Exactly how the climate will respond to this anthropogenic forcing is currently uncertain because our understanding of the climate system is incomplete. There are the things, however, we know we know. For instance, that as we increase the concentration of CO2 in the atmosphere the climate will warm [3]. Then there are the things we know we do not know, such as the exact value of climate sensitivity and the extent to which it depends on background climate state (e.g. [4]). Then there are the things we know nothing about—the unknown unknowns—which have the potential to take future climate into unimagined directions. Much of the research into predicting future warming involves the use of complex numerical models, climate models, which encapsulate the state-of-the-art understanding of the modern climate system. While these tools can inform on the ‘known unknowns’, albeit imperfectly as these are often too uncertain to parametrize directly or are emergent properties of the model that are hard to test against observations, they are completely blind to the unknown unknowns as these are by definition not quantitatively represented in any model. Consequently, we urgently need to find alternative tools other than models to investigate them.

Continue reading ‘Placing our current ‘hyperthermal’ in the context of rapid climate change in our geological past’

Capturing the global signature of surface ocean acidification during the Palaeocene–Eocene Thermal Maximum

Geologically abrupt carbon perturbations such as the Palaeocene–Eocene Thermal Maximum (PETM, approx. 56 Ma) are the closest geological points of comparison to current anthropogenic carbon emissions. Associated with the rapid carbon release during this event are profound environmental changes in the oceans including warming, deoxygenation and acidification. To evaluate the global extent of surface ocean acidification during the PETM, we present a compilation of new and published surface ocean carbonate chemistry and pH reconstructions from various palaeoceanographic settings. We use boron to calcium ratios (B/Ca) and boron isotopes (δ11B) in surface- and thermocline-dwelling planktonic foraminifera to reconstruct ocean carbonate chemistry and pH. Our records exhibit a B/Ca reduction of 30–40% and a δ11B decline of 1.0–1.2‰ coeval with the carbon isotope excursion. The tight coupling between boron proxies and carbon isotope records is consistent with the interpretation that oceanic absorption of the carbon released at the onset of the PETM resulted in widespread surface ocean acidification. The remarkable similarity among records from different ocean regions suggests that the degree of ocean carbonate change was globally near uniform. We attribute the global extent of surface ocean acidification to elevated atmospheric carbon dioxide levels during the main phase of the PETM.

Continue reading ‘Capturing the global signature of surface ocean acidification during the Palaeocene–Eocene Thermal Maximum’

Ocean acidification can interact with ontogeny to determine the trace element composition of bivalve shell

We sought to determine how pCO2 will affect the incorporation of trace elements into bivalve shell. This was to validate that under high pCO2 conditions reconstruction of animal movements is still viable; and to investigate potential trace element proxies for ocean carbonate chemistry. Here, we examined shell of the bivalve Perna canaliculus formed under current CO2 (pCO2 = 400 μatm) conditions and those predicted to exist in 2100 (pCO2 = 1050 μatm). Seventeen trace element:calcium ratios were examined at two locations within shells. Elements that are typically most useful in determining connectivity patterns (e.g., Sr, Mn, Ba, Mg, B) were not affected by pCO2 in shell produced early in individual’s lives. This suggests that the effects of ocean acidification on dispersal signatures may be dampened. However, cobalt, nickel, and titanium levels were influenced by pCO2 consistently across shells suggesting their role as potential indicators of CO2 level.

Continue reading ‘Ocean acidification can interact with ontogeny to determine the trace element composition of bivalve shell’

Lower Triassic deep sea carbonate precipitates from South Tibet, China

Sea-floor carbonate precipitates (SCPs), commonly seen in pre-Cambrian strata, were widely developed during the Permian–Triassic mass extinction and the Early Triassic recovery interval. Most SCPs are found in shallow water facies, with few SCPs reported from deep sea settings. Here, we document Lower Triassic deep sea SCPs from turbidite deposits exposed at the Xiukang section, South Tibet, China. The SCPs only occur within thin-bedded, silty limestones that are embedded in dark grey shale. Parallel–aligned mud cobbles, rounded micritic intraclasts and micro-erosional surfaces are commonly seen in these planar laminated limestones that contain abundant radiolarians and thin-shelled bivalves, indicating deposition in a deep basin via turbidity currents. The deep sea SCPs, which are comprised of bladed calcite crystals, display a weak vertical δ13C variation with the overlying matrix, and a uniform element distribution and consequently have a homogenous cathodoluminescence pattern, suggesting rapid precipitation in the open ocean, followed by a quick burial, which led to minimal diagenetic effects on the SCPs. We propose a new hypothesis that rapid carbonate precipitation resulted from mixing, driven by the turbidity current that introduced shallow seawater to the deep seawaters with a high alkalinity. Based on this model, the turbidity currents during the Early Triassic might serve as a potential role in the connection between the oxic surface oceans and the euxinic, deep oceans.

Continue reading ‘Lower Triassic deep sea carbonate precipitates from South Tibet, China’


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Ocean acidification in the IPCC AR5 WG II

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