Sr isotope ratios of modern carbonate shells; good and bad news for chemostratigraphy

The reconstruction of past variations in seawater strontium isotope ratios (87Sr/86Sr) from marine carbonate skeletons (MCSs) is based on the assumption that the Sr isotope composition of oceans is homogeneous worldwide. However, fossil materials are mainly from shelf domains whose modern Sr budget...

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Bibliographic Details
Published in:Geology (Boulder) 2018-11, Vol.46 (11), p.1003-1006
Main Authors: El Meknassi, S, Dera, G, Cardone, T, de Rafélis, M, Brahmi, C, Chavagnac, V
Format: Article
Language:English
Subjects:
accuracy
alkaline earth metals
Applied geology
Basins
Bivalvia
Calibration
Carbonates
Cenozoic
chemostratigraphy
Coastal waters
Domains
Earth Sciences
Endorheic basins
Evaluation
Fjords
Fluxes
Fossils
Freshwater
Gastropoda
Gastropods
geochemistry
Geology
global
Groundwater
Groundwater discharge
hydrochemistry
Inland water environment
Isotope composition
Isotope ratios
Isotopes
Lagoons
Marine conservation
Marine ecology
Marine environment
Marine molluscs
metals
modern analogs
Mollusca
Mollusks
Neogene
Oceans
Offsets
paleoenvironment
Phanerozoic
Quaternary
Ratios
reconstruction
Sciences of the Universe
sea water
Seawater
Shells
Sr-87/Sr-86
stable isotopes
Stratigraphy
Strontium
Strontium 87
Strontium isotopes
Tertiary
Weathering
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Description
Summary:The reconstruction of past variations in seawater strontium isotope ratios (87Sr/86Sr) from marine carbonate skeletons (MCSs) is based on the assumption that the Sr isotope composition of oceans is homogeneous worldwide. However, fossil materials are mainly from shelf domains whose modern Sr budget is known to be very sensitive to continental Sr fluxes. Their reliability as chemostratigraphic tools or weathering proxies may thus be questioned, especially because no formal appraisal of biological and environmental biases has been done on a global scale. Here, we provide new modern MCS 87Sr/86Sr values from various marine environments that we analyze statistically in regards to extensive compilations of MCS and water data. We show that most MCS 87Sr/86Sr values fall within the range of those of modern oceanic waters (0.709172 ± 23). Others, concerning ∼30% of studied localities, may display slight to considerable offsets compared to the 87Sr/86Sr signature of modern seawater (±250 × 10-6). These patterns mainly concern bivalves and gastropods, or more generally epibenthic and eurytopic organisms, from coastal domains with water mass restriction, low salinity, or strong continental supplies (fjords, lagoons, endorheic basins). Beyond classical fluvial freshwater influences, submarine groundwater discharges may lead to a less radiogenic 87Sr/86Sr signature of MCSs even in lagoons with euhaline conditions (e.g., Oualidia lagoon, Morocco). All biases considered, this calls for caution in relative dating, as we calculate that half the values of the Phanerozoic 87Sr/86Sr curve cannot provide time calibration accuracy better than ±3 m.y., which is necessary to discriminate the shorter stages.
ISSN:0091-7613
1943-2682
DOI:10.1130/G45380.1