The carbon isotopic composition of CAMP basalts

In the geologic record evidence for short-lived and thus devastating perturbations of the global C-cycle are provided by sudden negative carbon-isotope excursions (CIE’s) in the ocean-atmosphere system. A well known example is the so-called initial CIE which occurs worldwide shortly before the Triassic-Jurassic boundary. According to most researchers, the marked size of this shift (up to -8‰) suggests that, besides volcanogenic CO2 emissions from Central Atlantic magmatic province (CAMP) basalts, methane from destabilization of ocean floor clathrates (δ13C -60‰) or from contact metamorphism of organic-rich sediments (δ13C -35‰ to -50‰) may have had a dominant role. This interpretation is based upon the assumption that basaltic CO2 has a δ13C of approximately -5‰, comparable to that measured on present-day basaltic volcanoes. In order to test this latter hypothesis, we measured the bulk carbon isotopic composition of CAMP basalts and gabbros using sealed tube combustion. The preliminary bulk δ13C of five CAMP basalts are in the range -26‰ to -29‰ while one basalt yields δ13C of -14‰. However, this latter basaltic lava also has a high C content (1285 ppm), unlike the other basaltic lavas (C = 73-251 ppm) and intrusives (365-596 ppm). The anomalous basalt sample is quite altered suggesting that its more positive carbon-isotopic composition (-14‰) may be attributed to the presence of secondary calcite. The strongly negative composition of the other basalts and gabbros is more puzzling and may indicate contribution from soil-derived organic matter carbon (δ13C = -22 to -25; Ekart et al., 1999), although it seems surprising that such a process would result in similar carbon isotopic compositions for apparently unaltered basic rocks coming from different geographic regions (e.g., central Brazil, Sierra Leone, Morocco, Portugal) and crustal depths. Whilst we presently can’t constrain where the C is residing in the basalts, we note that the strongly negative δ13C of CAMP basalts are consistent with some previously published δ13C values for other LIP basalts (Hansen, 2007) and for mantle rocks (Deines, 2002) indicating that a primary origin of the low δ13C signature can’t be excluded.