Influence of diagenesis on the stable isotopic composition of biogenic carbonates from the Gulf of Tehuantepec oxygen minimum zone

Abstract

[1] In order to evaluate the influence of diagenetic and post-sampling processes on the stable oxygen and carbon isotope compositions of biogenic carbonates, we conducted a multiproxy study of organic-rich sediments from the eastern Pacific oxygen minimum zone. Core MD02-2520, which was retrieved from the Gulf of Tehuantepec (Mexico), has seasonal laminations and covers the last 40 kyr. Together with the presence of gypsum crystals and inorganic calcite aggregates, the occurrence of large excursions in the stable oxygen and carbon isotope records of both planktonic and benthic foraminifera (as large as +3‰ in d18 O and À5‰ in d 13 C) point to significant secondary transformations. Storage-related gypsum precipitation was ruled out since it implies sulfide reoxidation by oxygen that triggers biogenic calcite dissolution, which proved to be of minor importance here. Instead, precipitation of authigenic calcite during early diagenesis appears to be the most likely process responsible for the observed isotopic excursions. The d 13 C composition for inorganic calcite aggregates (À5 to À7‰) suggests a major contribution from anaerobic oxidation of organic matter. The d 34 S composition for gypsum crystals (À10 to +15‰) suggests a major contribution from anaerobic reoxidation of authigenic sulfides, potentially involving reactions with metal oxides and sulfur disproportionation. A minor part of the gypsum might possibly have formed as a result of local pore water salinity increases induced by gas hydrate formation.

Paleoceanographic evolution of the Gulf of Tehuantepec (Mexican Pacific) during the last ~6 millennia

“…According to the ages determined for each dated level, a quasi-constant sedimentation rate is apparent in Figure 4 with an average value of 1.1 mm/year. This result is consistent with the parallel sediment core MD02-2520, with an average sedimentation rate of 1 mm/year (Blanchet et al, 2012). However, it varies between dated levels showing the lowest value (0.8 mm/year) between 3332 and 1982 cal yrs BP immediately preceded by the highest rate (1.4 mm/year) between 4065 and 3332 cal yrs BP (Supplemental Appendix 2, Figure 4).…”

“…The distinctive feature of those samples missing benthic and planktonic foraminifera is the presence of (large) fragments of gypsum and/or pyrite (only in two samples). Precipitation of gypsum is caused by sulfide oxidation reactions under low oxygen conditions (Blanchet et al, 2012; Schnitker et al, 1980; Self-Trail and Seefelt, 2005). These reactions may cause the environment to turn more acidic, which can trigger the dissolution of biogenic carbonates (Schnitker et al, 1980).…”

Northern and southern hemisphere controls on seasonal sea surface temperatures in the Indian Ocean during the last deglaciation

“…According to the SEM photographs, recrystallization is also unlikely (Figure ). Additionally, G. ruber δ 13 C shows typical glacial‐interglacial changes with minimum values of ~0.4‰ (supporting information Figure 3), one value much too high to be explained by syn‐sedimentary recrystallization of pore water δ 13 C with negative δ 13 C DIC [ McCorkle et al ., ; Blanchet et al ., ]. Rather, these G. ruber δ 13 C shifts can be linked to changes in the global oceanic δ 13 C of the ΣCO 2 pool, which are triggered by changes in the terrestrial biosphere [ Shackleton , ].…”

Particulate nutrient fluxes over a fringing coral reef: Source‐sink dynamics inferred from carbon to nitrogen ratios and stable isotopes

“…by assuming a representative d 13 C value for biogenic carbonate, d 13 C CaCO 3 , e.g., approximately 26% (Blanchet et al 2012). Calculations were performed on atom percent data (e.g., % 13 C) rather than the d values calculated from Eq.…”