CE31 - Physique Subatomique, Sciences de l'Univers, Structure et Histoire de la Terre

Oxygen Isotope ratios of Mesozoic Oceans Revisited – Oxymore

What can fossils from the time of the dinosaurs tell us about climate change?

Towards a robust quantification of climate conditions during the Mesozoic Era

The stable oxygen isotope ratio of Mesozoic oceans, a key parameter to better understand the climate machine

The Mesozoic era (-250 Ma to -66 Ma) records some of the highest atmospheric carbon dioxide (CO2) levels in Earth's history. Our knowledge of climatic conditions during this period is based mainly on measurements of the ratio of oxygen 18 to oxygen 16 recorded in marine fossils. However, this ratio varies not only with temperature but also with the availability of these two oxygen isotopes in seawater. Thus, the lack of robust estimates of the ratio of oxygen 18 to oxygen 16 in seawater for this time interval strongly limits reconstructions of Mesozoic climate, and thus our understanding of past climate sensitivity to variations in CO2 concentrations. The aim of this project is to provide the first reliable estimates of the spatiotemporal evolution of the Mesozoic ocean oxygen 18/oxygen 16 ratio in order to test the following hypotheses: 1) marine temperatures in the Mesozoic were not particularly warm, implying that factors other than CO2 control the Earth's climate on geological timescales; 2) the Mesozoic climate was very warm and characterized by very weak latitudinal temperature gradients, implying a higher climatic sensitivity than currently assumed by climate models; 3) temperatures and sea level changed rapidly and globally during this interval as a result of glacial-interglacial cycles.

The OXYMORE project is based on three complementary axes to reconstruct the spatial and temporal variations in the Mesozoic oceans' oxygen 18/oxygen 16 ratio using:

1) geochemical measurements carried out on marine invertebrate fossils, which will provide independent paleo-thermometers. These fossils are mainly sampled from academic collections and supplemented by fieldwork for three key time intervals.

2) oxygen isotope measurements carried out on fossil remains of endothermic («warm-blooded«) marine animals. These animals, capable of maintaining a constant body temperature, thus incorporate oxygen isotopes according to the oxygen 18/oxygen 16 ratio of the oceans. This approach requires detailed isotope mapping of modern (cetaceans) and Mesozoic (ichthyosaurs and plesiosaurs) skeletons of marine endotherms to identify skeletal elements least affected by changes in external temperatures. Systematic isotopic measurements of the selected elements of different ages and regions will then be carried out using museum collections and fieldwork.

3) The use of modern ocean isotope and climate databases and numerical models. These simulations will be used to study the impact of palaeogeography on the isotopic composition of the ocean for four key intervals. In particular, these simulations will allow a detailed examination of the hydrological and isotopic dynamics of the Mesozoic shallow seas, where most of the fossils found today were deposited.

The expected results of this project will shed an ancient light on the Earth's climate sensitivity, with obvious but fundamental implications for global environmental and economic policies.

NA.

Séon N, Amiot R, Martin J, Young M, S., Middleton H, Fourel F, Picot L, Valentin X and Lécuyer C (2020). Thermophysiologies of Jurassic marine crocodylomorphs inferred from the oxygen isotope composition of their tooth apatite. Philosophical Transactions of the Royal Society B: Biological Sciences 375(1793). doi 10.1098/rstb.2019.0139

The Mesozoic Era (–250 Ma to –66 Ma) records some of the highest atmospheric CO2 levels of Earth’s history. The lack of robust estimates of the oxygen isotope ratios of seawater (d18OSW) for this interval, however, strongly limits Mesozoic climate reconstructions and hence our understanding of past climate sensitivity to changing CO2 concentrations. The aim of this project is to provide the first reliable Mesozoic d18OSW estimates using three complementary approaches, in order to verify or falsify the following hypotheses: 1) the Mesozoic had “low” d18OSW ratios, implying that its climate was not particularly warm and that other factors than CO2 control Earth’s climate on geological timescales; 2) the Mesozoic had “high” d18OSW ratios, implying very warm conditions and higher-than-expected climate sensitivity; 3) d18OSW ratios changed rapidly and globally during this interval in concert with sea-level changes as a result of greenhouse-icehouse cycles. We propose three complementary axes to test these hypotheses:
In a first axis, we will compare skeletal d18O values with two different paleotemperature proxies (primarily Mg/Ca ratios; delta47 on selected samples), measured on the same brachiopod specimens, to reconstruct d18OSW values at multiple locations through the Mesozoic. The fossils will be predominantly sampled from academic collections and completed by fieldwork for key, selected time intervals and their preservation will be assessed using well-proven criteria.
In a second axis, we will measure d18O values of skeletal remains of marine endotherms, which incorporate 18O and 16O primarily as a function of d18OSW values, to trace temporal and spatial changes in d18OSW values through the Mesozoic. We will first map intra-skeletal d18O variability in both modern (cetaceans) and Mesozoic marine endotherms (i.e., ichthyosaurs and plesiosaurs) to identify the skeletal elements less affected by regional heterothermy (i.e., cooler body extremities used to reduce heat loss to their highly conductive aquatic environment) and therefore able to more reliably track past d18OSW values. A systematic investigation of d18O values of the selected fossil skeletal elements of different age and paleolatitude will then be performed using both museum collections and fieldwork.
In a third axis, Mesozoic d18OSW values of surface and deep oceans will be simulated using modern d18OSW databases and General Circulation Models (GCM). These simulations will be used to investigate the impact of paleogeography on salinity and d18OSW values in three selected Mesozoic time slices, and notably to critically examine the hypothesis according to which high freshwater input in Mesozoic shallow seas, which constitute the depositional environment of the bulk of strata of this age now available for skeletal d18O measurements, produced very low d18OSW values, leading to strong paleotemperature overestimates.
The results will be integrated to estimate spatial (axes 1+2+3) and temporal changes (axis 1+2) in d18OSW on both long-term and short-term timescales (i.e., during events of major environmental change) during the Mesozoic and hence examine critically the three, above-mentioned key hypotheses. The expected results of this project will shed a long overdue, ancient light on Earth’s climate sensitivity, with obvious but fundamental implications for global environmental and economical policies.

Project coordinator

Monsieur Guillaume SUAN (Laboratoire de géologie de Lyon : Terre, planètes et environnement)

The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.

Partner

GET Géosciences Environnement Toulouse
LSCE Laboratoire des Sciences du Climat et de l'Environnement
CR2P Centre de recherche sur la paléobiodiversité et les paléoenvironnements
CNRS DR12 - CEREGE Centre National de la Recherche Scientifique - Centre européen de recherche et d'enseignement de géosciences de l'environnement
LGL-TPE Laboratoire de géologie de Lyon : Terre, planètes et environnement

Help of the ANR 500,498 euros
Beginning and duration of the scientific project: January 2019 - 48 Months

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