Nd isotopes of ODP Sites 146-893 and 167-1017 samples

Changes in the source of intermediate waters to the southern California margin may have caused variations in seafloor oxygen levels on stadial–interstadial time scales. We test this hypothesis using the Nd isotopic composition of benthic foraminifera and fossil fish debris from ODP Sites 893 and 1017 to track the composition of intermediate waters across interstadials 8-14 (~37-52 ka) during Marine Isotope Stage 3. The epsilon-Nd values of waters bathing the seafloor at Site 893 were typically ~-9 and those bathing Site 1017 were ~-7, both of which are significantly less radiogenic than waters that had originated in either the North Pacific or Southern Ocean (by the time such waters reached the southern California margin). Detrital silicate epsilon-Nd values of nearly -12 suggest that this offset toward lower epsilon-Nd values was likely caused by boundary scavenging that partially overprinted the water mass composition with local/regional fluvial Nd inputs. In spite of the evidence for boundary scavenging, the lack of systematic seawater Nd isotope changes on a stadial–interstadial basis suggests that the provenance of the intermediate waters did not change, and that the waters were derived from the Southern Ocean. Instead, changes in local/regional sea surface productivity may have caused the recorded changes in seafloor oxygenation.加利福尼亚南部陆缘中层水源的变化，可能引发了冰阶-间冰阶时间尺度下的海底氧含量波动。我们借助大洋钻探计划（Ocean Drilling Program, ODP）893与1017站位的底栖有孔虫和鱼类化石碎屑的钕同位素组成，对这一假说开展检验，以追踪海洋同位素第3阶段（Marine Isotope Stage 3）期间间冰阶8-14（约37~52 ka）的中层水组成。站位893处流经海底水体的εNd（epsilon-Nd）值通常约为-9，站位1017处则约为-7；二者的放射成因特征均显著弱于源自北太平洋或南大洋的水体（直至这些水体抵达加利福尼亚南部陆缘时）。近乎-12的碎屑硅酸盐εNd值表明，这种向更低εNd值的偏移，大概率由边界清除作用所致——该作用将局地/区域河流输入的钕信号叠加在了原有水团的组成之上。尽管存在边界清除作用的相关证据，但冰阶-间冰阶尺度下海水钕同位素并未出现系统性变化，这说明中层水源并未发生改变，且该水体源自南大洋。与之相反，局地/区域海面生产力的变化，可能是记录到的海底氧合作用变化的驱动因素。