High-resolution isotopic records (δ
18O and δ
13C) and cathodoluminescence study of lucinid shells from methane seeps of the Eastern Mediterranean |
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| Authors: | Cécile Lietard Catherine Pierre |
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| Institution: | (1) LOCEAN, Laboratoire d’Océanographie et du Climat: Expérimentation et Approches Numériques (UMR 7159), Université Pierre et Marie Curie, Case 100, 4 Place Jussieu, 75252 Paris cedex 05, France |
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| Abstract: | We present high-resolution isotopic records and cathodoluminescence studies of recently dead and live bivalve specimens from
cold seeps, in an attempt to reconstruct environmental conditions during organism growth, and thereby the possible variability
of fluid-venting activity at the seafloor. Shells of the burrowing lucinid Myrtea aff. amorpha were collected at three localities near actively venting methane seeps in the Eastern Mediterranean deep sea, using the Nautile
submersible during two French oceanographic cruises: from the Kazan mud volcano, in the vicinity of the Anaximander mounts
(MEDINAUT cruise, 1998), and from the central pockmark province and the Amon mud volcano of the Nile deep-sea fan (NAUTINIL
cruise, 2003). The oxygen and carbon isotope compositions of 18 shells from the various localities, and also from different
sites at the same locality show a rather strong scatter (1.8 < δ
18O‰ < 3.4; −10.2 < δ
13C‰ < 2.2), and values lower than those expected for carbonate precipitated at equilibrium with present-day bottom waters.
This means that warm methane-rich fluids were mixed with bottom seawater during precipitation of shell carbonates. We have
tried to determine ontogenetic age of two shells by using cathodoluminescence as a sclerochronological proxy, because the
direct counting of carbonate increments was not possible in these specimens. There is a relatively good correspondence between
cathodoluminescence trends and oxygen isotope profiles that might support the link between manganese incorporation during
growth and temperature. Eight specimens of lucinid shells were selected for high-resolution isotopic profiling. A few shells
exhibit decreasing δ
18O and δ
13C values from the umbo to the actively growing ventral shell margin, which can be attributed to the commonly observed physiologically
controlled deceleration of growth with increasing organism age, this metabolic effect corresponding to the increase of incorporation
of respiratory CO2. A few shells exhibit high-frequency δ
18O variations with an amplitude of about 1.5‰ that might be related to temperature variations controlled by fluid-venting activity.
One shell from the pockmark province of the Nile deep-sea fan records a strong, sharp δ
13C decrease of about 9‰, and extending over a 5-mm interval in the shell that can be related to a major methane release event.
Another shell from the Kazan mud volcano exhibits a progressive increase of δ
13C values from −10‰ to 0‰ with age, which might indicate decreasing methane flow throughout the organism’s life. This study
has demonstrated that bivalve shells from deep-sea cold seeps represent good indicators of variability in seepage activity
of methane-rich fluids, at various scales in both space and time. Although the precise chronology of the observed events was
not established, because shell growth rate is not known in this case, this remains a priority for future studies in such environments. |
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