Stable isotopes (d13C, d18O) on elevated epifauna from the Pliocene and Holocene

The presence and high abundances of the benthic foraminiferal group "elevated epifauna" has been proposed as indicator of the existence of Mediterranean Outflow Water (MOW) in the Gulf of Cadiz. Here we intend to use this potential proxy to reconstruct MOW in the early Pliocene at the Integrated Ocean Drilling Program (IODP) Hole U1387C. Cibicides lobatulus, Planulina ariminensis and Cibicides refulgens were found at this site, but high abundances of C. lobatulus and C. refulgens coincide with increases of shelf taxa and grain size, related to episodes of downslope transport. This issue calls into question the applicability of these species as MOW proxies. The present study therefore reassesses the role of these three elevated epifaunal species by means of stable isotope analyses (d18O, d13C) by contrasting their isotopic signature with that of shelf dwellers and deep-water taxa from Pliocene and Recent sediments from the Gulf of Cadiz and the western Iberian margin. Since foraminiferal calcite does not always precipitate in equilibrium with seawater, substantial efforts have been taken in order to determine species-specific offset values and to correct the isotopic signature for vital effects. Our corrected results demonstrate that C. lobatulus and C. refulgens are isotopically similar to the shelf dwellers in the Pliocene data-set, eliminating them as MOW indicators. Recent samples from the Gulf of Cadiz and the western Portugal shelf corroborate the wide bathymetric range of C. lobatulus from the shelf to the slope. Our results thus warrant for caution when considering C. lobatulus and C. refulgens as indicators of bottom current strength in unstable slope settings, particularly when co-occurring with allochthonous shelf dwellers. In contrast, P. ariminensis shows a signature close to deep-water taxa, corroborating its reliability as indicator of MOW.底栖有孔虫类群中的“高位附生类群（elevated epifauna）”的出现与高丰度，曾被提议作为加的斯湾存在地中海流出水（Mediterranean Outflow Water, MOW）的指示标志。本研究拟利用这一潜在替代指标，重建综合大洋钻探计划（Integrated Ocean Drilling Program, IODP）U1387C钻孔早上新世时期的地中海流出水演化历史。 该站位已发现圆辐虫（Cibicides lobatulus）、阿尔米诺平旋虫（Planulina ariminensis）与光亮圆辐虫（Cibicides refulgens），但圆辐虫C. lobatulus与光亮圆辐虫C. refulgens的高丰度，与陆架类群丰度升高及沉积物粒度变粗同步出现，这与多期斜坡搬运作用密切相关。这一现象对上述物种作为地中海流出水替代指标的适用性提出了质疑。 因此，本研究通过稳定同位素分析（δ¹⁸O、δ¹³C），重新评估这三种高位附生有孔虫的生态功能：将其同位素信号与加的斯湾及伊比利亚西部边缘上新世与现代沉积物中的陆架生物、深水类群的同位素信号进行对比。 由于有孔虫方解石并不总是与海水达到同位素平衡分馏，因此本研究已开展大量工作以确定物种特异性同位素分馏偏移值，并校正生命效应（vital effects）导致的同位素信号偏差。 校正后的结果显示，在上新世数据集内，C. lobatulus与C. refulgens的同位素信号与陆架生物高度相似，因此排除了其作为地中海流出水指示标志的可能性。 采自加的斯湾与葡萄牙西部陆架的现代样本进一步证实，C. lobatulus的水深分布范围极广，从陆架延伸至斜坡。因此，当将C. lobatulus与C. refulgens作为不稳定斜坡环境中底流强度的指示标志时，需格外谨慎，尤其是在其与异地陆架生物伴生的情况下。 与之相反，P. ariminensis的同位素信号与深水类群相近，证实了其作为地中海流出水指示标志的可靠性。