Strontium isotope, amino acid, and fossil taxonomy data to aid in identifying instances of marine terrace reoccupation on Anacapa and San Miguel Islands, California, USA

In areas of low uplift rate on the Pacific Coast of North America, reoccupation of emergent marine terraces by later high-sea stands has been hypothesized to explain the existence of thermally anomalous faunas (mixtures of warm and cool species) of last interglacial age. If uplift rates have been low for much of the Quaternary, it follows that higher (older) terraces should also show evidence of reoccupation. Strontium isotope analyses of fossils from a high-elevation marine terrace on Anacapa Island, California yield a suite of ages ranging from ~2.4-2.3 Ma to ~1.4-1.5 Ma. These results indicate that terrace reoccupation and fossil mixing on Anacapa Island could have taken place over several interglacial periods in the early Pleistocene. Terrace reoccupation over this time period is likely a function of both a low uplift rate and the timing of orbital forcing of glacial-interglacial cycles. Nearby San Miguel Island also has evidence of terrace reoccupation, with Sr-isotope ages of shells from several high-elevation terraces ranging from ~1.21-1.25 Ma to ~0.43-0.50 Ma. However, the frequency of terrace reoccupation was much lower than on Anacapa Island. The uplift rate of San Miguel Island is higher than that of Anacapa Island and the terraces formed when glacial-interglacial cycles were longer. The frequency of terrace reoccupation is controlled by the rate of tectonic uplift and the timing of orbital forcing of sea level change during glacial-interglacial cycles.

在北美太平洋海岸抬升速率较低的区域，学界曾提出假说：后期高海面期对出露海成阶地的重新占据，可解释末次间冰期热异常生物群（暖水与冷水物种混合）的存在。若第四纪多数时期抬升速率均处于较低水平，则高程更高、年代更古老的阶地也应留存被重新占据的证据。对加利福尼亚州阿纳卡帕岛一处高海拔海成阶地中的化石开展锶同位素分析，测得其年代范围约为2.4~2.3 Ma至1.4~1.5 Ma。上述结果表明，阿纳卡帕岛的海成阶地重新占据与化石混合现象，可能发生于更新世早期的多次间冰期之中。该时期内的阶地重新占据，可能同时受低抬升速率与冰期-间冰期旋回轨道驱动力的时间节点双重调控。邻近的圣米格尔岛同样存在阶地重新占据的证据，其多处高海拔阶地壳体的锶同位素年代范围约为1.21~1.25 Ma至0.43~0.50 Ma。但圣米格尔岛的阶地重新占据频率远低于阿纳卡帕岛。圣米格尔岛的构造抬升速率高于阿纳卡帕岛，且其阶地形成于冰期-间冰期旋回更长的地质时期。阶地重新占据的频率，受控于构造抬升速率以及冰期-间冰期旋回中海平面变化的轨道驱动力时间节点。