Neodymium isotopic record of planktonic foraminifera in sediment core MD81-LC21

A well developed sapropel (S5) was deposited in the eastern Mediterranean during the Last Interglacial (Eemian), 124-119 ka. Freshwater contributions to the basin at this time can be traced using the isotopic composition of Nd in planktonic foraminifera. This enables differentiation between radiogenic sources to the south, under the influence of the African monsoon, and unradiogenic sources to the north, relating to the mid-latitude westerlies. Here we compare new Nd data, from a core in the southeast Aegean Sea, with published data from the Ionian and Levantine Seas. Shifts towards more radiogenic Nd in the lower and middle parts of sapropel S5 are most pronounced in the Ionian Sea record, with epsioln-Nd and d18O G. ruber co-varying more closely here than in the Levantine and Aegean Seas. This is consistent with a freshwater source proximal to the Ionian Sea site, likely indicating a substantial reactivation of rivers flowing northward from the central Saharan watershed. The lack, during S5 deposition, of a noticeable shift towards more unradiogenic Nd in the Aegean record would exclude a large influx of water from the northern borders of the eastern Mediterranean during sapropel deposition. These findings support a scenario whereby the Last Interglacial eastern Mediterranean was influenced strongly by the remote effects of an intensified African monsoon, with more local precipitation in the northern borders contributing relatively little to the sea surface composition.在124~119 ka的末次间冰期（埃姆间冰期，Last Interglacial/Eemian），地中海东部沉积了发育完好的腐泥层S5（sapropel S5）。彼时注入该盆地的淡水信号，可通过浮游有孔虫（planktonic foraminifera）壳体中的钕（Nd）同位素组成进行示踪。该方法可区分受非洲季风影响的南部放射成因物源，以及与中纬度西风带相关的北部非放射成因物源。本研究将爱琴海（Aegean Sea）东南部岩芯的新钕同位素数据，与已发表的爱奥尼亚海（Ionian Sea）、黎凡特海（Levantine Sea）的相关数据开展对比分析。腐泥层S5中下部的钕同位素比值向更具放射成因的方向偏移，这一变化在爱奥尼亚海的记录中最为显著；该区域的εNd值与红拟抱球虫（G. ruber）的氧同位素（δ¹⁸O）值的协同变化程度，远高于黎凡特海与爱琴海区域。这一结果与爱奥尼亚海站位附近的淡水补给源特征相符，指示源自撒哈拉中部流域向北径流的河流发生了显著复活。在腐泥层S5沉积期间，爱琴海记录中未出现明显的非放射成因钕同位素偏移，这一现象排除了地中海东部北部边界在该时期有大量水体注入的可能性。上述研究结果支持如下情景：末次间冰期地中海东部受增强的非洲季风的远程影响主导，而北部边界的局地降水对海水表层组成的贡献相对有限。