Research paperCalculation of the reservoir age from organic and carbonate fractions of sediments in the Gulf of Cariaco (Caribbean Sea)
Introduction
The difference between radiocarbon ages of organisms belonging to the terrestrial biosphere and their contemporary marine equivalents was defined by Stuiver and Polach (1977) as the “reservoir age”. For aquatic (marine, lacustrine or mixed) organisms, in organic matter (OM) derived from photosynthesis, the amount of fixed 14C depends, on one hand, on its atmospheric production rate (which changes with the cosmic flux) and, on the other hand, on the 14C activity within the incorporated dissolved CO2 in the seawater. In turn, this 14C content depends on the CO2 residence time within the specific water mass and on the hard water effect related to 14C-free inorganic carbon originating from sub-aerial dissolution of old carbonate rocks (Spennemann and Head, 1998, Sabatier et al., 2010). The delay between dissolution and incorporation into marine OM is attributable to different local influences, e.g., water depth, circulation, surface mixing, etc.
When the age difference between a marine sample and its atmospheric “stratigraphic” equivalent is linked to marine 14C variations in response to CO2 exchanges between the atmosphere and the ocean, this effect may be corrected by subtracting a value, Rt, derived from modeled ocean average (Reimer et al., 2013) from the conventional marine 14C age. However, several studies have suggested the possibility of significant deviations in the regional marine reservoir signature from this average value (Folk and Ward, 1957, Goodfriend and Flessa, 1997, Siani et al., 2001, Reimer and McCormac, 2002, Southon et al., 2002, Sabatier et al., 2010). This ΔR value represents the radiocarbon exchange conditions between the atmosphere and a specific reservoir and can be quantified by the difference between the regional marine and the global marine 14C ages (Stuiver and Braziunas, 1993).
For conventional ages derived from the particulate (bulk) organic fraction from gulf and estuary sediments, the corrections of overestimated ages due to this effect are not clear and can be linked to i) possible mixing of reworked fluvial and marine organic matter (Ingram and Southon, 1996, Goodfriend and Flessa, 1997), ii) the hard water effect (Spennemann and Head, 1998), iii) upwelling processes (Stuiver and Braziunas, 1993), and iv) carbon sequestration in sediments and carbonate from the methanogenesis (Aharon and Gupta, 1994, Luff and Wallmann, 2003, Orphan et al., 2004, Naehr et al., 2007, Paull et al., 2007).
In the case of the Gulf of Cariaco, no local reservoir effect has previously been reported, neither for the sediment bulk organic fraction (SBOF) nor for carbonate shells. We here present a set of 24 radiocarbon dates taken from two cores collected from the deepest part of the gulf, which exhibits anoxic conditions. We aim to establish an accurate chronology for the last centuries in order to detect traces of historical earthquakes and tsunami (Aguilar et al., 2016).
Section snippets
Study site
The Gulf of Cariaco, an appendix of the Cariaco Basin (or Cariaco Trough), is an E-W-elongated semi-closed basin that is 60 km long and 15 km wide, with an average depth of 55 m. It is located in northeastern Venezuela, on the southeastern boundary of the Caribbean realm (Fig. 1).
Geological setting
The Cariaco Basin is a pull-apart basin that relays slip between the San Sebastian Fault (West) and the El Pilar Fault (East). The Gulf of Cariaco has formed directly above the latter fault (Fig. 1). More precisely, the El Pilar dextral strike slip fault (EPF) is represented by the VE-13b segment, whose submarine morphologic expression is approximately 50 km long through the gulf (Audemard et al., 2007). The bottom of the gulf is characterized by two major features (Fig. 1):
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Along the southern
Hydrographic setting
In addition to the above-mentioned lithological contrasts, there are also clear differences in the hydrographic network. Along the northern coast, the tributaries of the gulf are short and experience intermittent flooding (Febres-Ortega, 1974, Caraballo, 1982b, Márquez et al., 2005, Quintero et al., 2005). Due to the low quantity of precipitation on the Araya Peninsula, theses rivers have a minor contribution to the whole hydrological input into the Gulf of Cariaco (Márquez et al., 2011). On
Sampling and sedimentary analyses
Two short gravity cores (approximate 1 m) were selected from among a set of 18 cores. The studied cores were collected from the Guaracayal depression – Cariac-09-06 (10°28′29″N; 63°57′58″W) and Cariac-09-05b (10°18′19″N; 63°58′57″W) – in 2009 at a water depth of 90 m (Fig. 1) while aboard the R/V GUAÏQUERI II. In the laboratory, the cores were cut into two halves. Each half-section was described in detail and pictures were taken. The lithological description of the sequences allowed the
Main sedimentary features
The two cores display a uniform olive green silty sediment that is darker than most detrital sand layers. Based on observations of grain size, LOI analysis and smear slide descriptions, two facies have been defined:
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Facies 1: This facies is present in all cores. Its color is predominantly olive gray (5Y3/2) and is rich in organic matter (≃19 % wt). The particle size distribution analyses (Fig. 3) show a dominance of silt-sized particles from 15 to 30 μm (Cariac-09-06: 70.0 ± 4.5%; Cariac-09-05B:
Radiocarbon anomalies
The ages of most of the surficial samples (shallower than 15 cm core depth) will not be considered in this calculation. The modern organic carbon content in the sample of marine origin collected within the uppermost 15 cm implies that the sediments located at this depth may potentially record an increase in the 14C activity associated with the post-bomb peak. Consequently, the global and local reservoir effect correction would be wrong.
The facies 2 sediments (grey bands in Fig. 5) exhibit an
Conclusions
The analyses presented in this work provide a ΔR estimation that can be used for the correction of 14C ages of SBOF samples from the Gulf of Cariaco. Our results indicate that, despite the effect of a significant upwelling regime, the 14C ages of different carbonate samples (benthic and planktonic) do not record a local reservoir effect (ΔR = 0). The average ΔR value derived from the 14C measurements of the sediment bulk particulate organic fraction was successfully applied to produce a
Acknowledgements
The Gulf of Cariaco coring survey performed in 2009 is part of the Venezuelan-French ECOS-Norte grant No V10U01, entitled “Contribution to multi-risk assessment - tsunamis, earthquakes, extreme-weather events - along the edge Caribbean of Venezuela, through a combined tectonic sedimentological and morphodynamic approach”. We thank FUNVISIS, CNRS-ISTerre, CNRS-BRGM-ISTO and CNRS-EDYTEM laboratories for cooperation and laboratory facilities. Our investigations also benefited from the CNRS-INSU
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