Benthic oxygen isotopes, Zr/Al and grain size fractions over the Miocene-Pliocene boundary in the Gulf of Cadiz@en

During the latest Messinian, hemipelagic sediments exhibiting precession-induced climate variability were deposited. These are overlain by Pliocene sediments deposited at a much higher sedimentation rate, with much higher and more variable XRF-scanning Zr/Al ratios than the underlying sediment, and that show evidence of winnowing, particle sorting and increasing grain size, which we interpret to be related to the increasing flow of MOW. Pliocene sedimentary cyclicity is clearly visible in both the benthic d18O record and the Zr/Al data and is probably also precessionally controlled. On the basis of these results, we conclude that contouritic sedimentation, associated with weak Mediterranean-Atlantic exchange, began in the Gulf of Cadiz virtually at or shortly after the Miocene-Pliocene boundary, with two contouritic bigradational sandy-beds within the fourth precession cycle after the Miocene-Pliocene boundary.在最新墨西拿期（latest Messinian），沉积了呈现岁差驱动气候变率特征的半远洋沉积物（hemipelagic sediments）。该套沉积物之上覆有沉积速率显著更高的上新世（Pliocene）沉积物，其X射线荧光扫描（XRF-scanning）获取的锆铝比值（Zr/Al ratios）不仅数值更高，且波动幅度更大；同时该套沉积物显示出淘洗作用（winnowing）、颗粒分选（particle sorting）及粒度（grain size）逐渐增大的证据，我们将其解释为与地中海溢流流水体（MOW, Mediterranean Overflow Water）流量增加相关。上新世沉积旋回在底栖有孔虫δ¹⁸O记录（benthic d¹⁸O record）与锆铝比值数据中均清晰可见，其大概率同样受岁差周期（precession cycle）调控。基于上述研究结果，我们得出结论：与弱地中海-大西洋水体交换（Mediterranean-Atlantic exchange）相关的等深流沉积（contouritic sedimentation），实际上恰在中新世-上新世边界（Miocene-Pliocene boundary）或其之后不久就开始发育于加的斯湾（Gulf of Cadiz）；在中新世-上新世边界后的第四个岁差周期内，存在两层等深流成因双渐变砂层（contouritic bigradational sandy-beds）。