Geochemical signatures in Late Triassic brachiopods from New Caledonia

Brachiopod fossils from the sedimentary sequences of the Téremba Terrane (New Caledonia) provide a unique opportunity to study the environmental parameters of the Late Triassic. ⁸⁷Sr/⁸⁶Sr ratios, δ¹³C and δ¹⁸O values, and Sr/Ca, Mg/Ca and Mn/Ca ratios were measured on brachiopods from Oretian to Otapirian (Norian to Rhaetian) fossil localities of the Baie de St.-Vincent area. Post-depositional impacts on the geochemical proxies were investigated by analysing calcite cements and partly recrystallized shell material. Diagenetic fluids carried strontium with a low ⁸⁷Sr/⁸⁶Sr ratio of c. 0.7065, and light δ¹³C values of c. −20‰, suggesting major contributions by oxidized organic matter. Diagenetic equilibrium was reached at δ¹⁸O values of c. −12‰, very low Sr/Ca ratios of < 0.05 mmol/mol and locally variable Mn/Ca ratios of up to 5.9 mmol/mol. Results from the best-preserved samples suggest that calcification temperatures of Warepan and Otapirian brachiopods from New Caledonia were consistently ≥ 15 °C.

产自新喀里多尼亚特伦巴地体（Téremba Terrane）沉积序列中的腕足动物化石（brachiopod fossils），为晚三叠世环境参数的研究提供了独特契机。研究人员针对圣文森特湾（Baie de St.-Vincent）地区奥列蒂安期（Oretian）至奥塔皮里安期（Otapirian，对应诺利期（Norian）至瑞替期（Rhaetian））的化石产出点位，开展了腕足化石的⁸⁷Sr/⁸⁶Sr比值、δ¹³C及δ¹⁸O值，以及Sr/Ca、Mg/Ca、Mn/Ca比值的测定工作；通过分析方解石胶结物与部分重结晶的壳体物质，探究了沉积后作用对地球化学代用指标的影响。成岩流体携带的锶具有约0.7065的低⁸⁷Sr/⁸⁶Sr比值，以及约-20‰的轻δ¹³C值，表明其主要来源于氧化有机质；当δ¹⁸O值约为-12‰时体系达到成岩平衡，此时Sr/Ca比值极低（<0.05 mmol/mol），Mn/Ca比值存在局部波动，最高可达5.9 mmol/mol。保存状况最优的样品结果显示，产自新喀里多尼亚的Warepan期与奥塔皮里安期腕足动物的钙化温度始终≥15℃。