Sedimentological, micropaleontological and stable isotope investigation of the lower Oligocene in DSDP Core 40-363 from Walvis Ridge, southeastern Atlantic (Table 1)

The recurrence of Braarudosphaera chalks in the lower Oligocene sequences of the subtropical South Atlantic has been a long-standing conundrum, with many hypotheses having been advanced to explain the genesis of these exotic nannofossil assemblages. Here, we evaluate different paleoceanographic models within the context of stable isotope (delta18O, delta13C) data measured from bulk-sediment samples and well-preserved foraminifera. Two closely-spaced Braarudosphaera layers from a lower Oligocene (foram Subzone P21a, 29.4-28.5 Ma) section drilled in the southeastern Atlantic (DSDP Site 363) are investigated. Maximum durations for the blooms that deposited the lower and upper Braarudosphaera layers are estimated to be 1.1 and 2.2 k.y., respectively. Bulk-sediment samples enriched in braarudosphaerid carbonate exhibit pronounced delta18O increases on the order of 0.6-1.0‰ which we attribute to isotopic disequilibria driven by braarudosphaerid vital effects. The two Braarudosphaera layers straddle a single peak in benthic foraminiferal delta18O values, suggesting that these blooms may recur on glacial/interglacial timescales. This same pair of braarudosphaerid layers also occurs as a couplet bundled with prolonged (~6.7 k.y.) thermocline cooling, evidence that these stratigraphically distinct deposits may represent a 'split signal' for a single paleoceanographic/paleoclimatic event. Subsumed within this episode of subsurface cooling are two short-lived, negative excursions (~0.5‰) in the delta13C record of a thermocline-dwelling planktonic foraminifer that coincide with the braarudosphaerid layers. Thus, benthic-to-thermocline delta18O and delta13C gradients were reduced during the braarudosphaerid blooms, a hallmark signature for strengthened upwelling. Both braarudosphaerid layers are marked by transient divergences in the stable isotopic signals of two shallow-dwelling species of planktonic foraminifera. These transient delta18O offsets may reflect subtle differences in the depth ecologies of these two mixed-layer species. If so, then braarudosphaerid depositional events may represent 'subsurface blooms' that took place within the lower parts of the euphotic zone. Alternatively, these transient delta18O offsets may reflect periods of pronounced seasonality, with braarudosphaerid blooms occurring during spring upwelling. The recurrence of Braarudosphaera blooms on both sides of the South Atlantic is believed to reflect rhythmic changes in the vigor and configuration of gyre circulation. We speculate that the termination of Braarudosphaera blooms in the South Atlantic near the end of the Early Oligocene may be related to paleoceanographic change caused by the crossing of a critical threshold in the tectonic opening of the Drake Passage and the development of the Antarctic Circum-Polar Current.

南大西洋亚热带地区渐新统下部地层中Braarudosphaera白垩（Braarudosphaera chalks）的反复出现长期以来都是一个悬而未决的科学难题，学界已提出诸多假说来解释这类奇特的超微化石组合（nannofossil assemblages）的成因。本研究基于从bulk沉积物样品（bulk-sediment samples）以及保存完好的有孔虫（foraminifera）中测得的稳定同位素（delta18O、delta13C）数据，对不同古海洋学模型展开评估。研究对象为南大西洋东南部深海钻探计划（Deep Sea Drilling Project，DSDP）363站位所钻取的渐新统下部（有孔虫亚带P21a，29.4~28.5 Ma）地层中两处间距较近的Braarudosphaera层。经估算，沉积形成上下两层Braarudosphaera的浮游生物水华（blooms）的最大持续时长分别为1.1千年和2.2千年。富含Braarudosphaerid碳酸盐的bulk沉积物样品显示出显著的δ18O（delta18O）偏移，幅度达0.6‰~1.0‰，我们将其归因于Braarudosphaerid生命效应（vital effects）驱动的同位素不平衡现象。两层Braarudosphaera地层横跨底栖有孔虫δ18O值的单个峰值，表明这类水华可能以冰期/间冰期时间尺度循环出现。该组Braarudosphaerid地层还以耦合沉积层的形式出现，并伴随持续约6.7千年的温跃层（thermocline）降温，这一证据表明，这些地层学上独立的沉积体可能代表了单次古海洋/古气候事件的"拆分信号"。在此次地下降温事件中，栖息于温跃层的浮游有孔虫的δ13C（delta13C）记录出现两次短暂的负偏移（幅度约0.5‰），且与Braarudosphaerid地层的出现时间相吻合。因此，在Braarudosphaerid水华发生期间，底栖-温跃层δ18O和δ13C梯度会出现降低，这正是上升流增强的标志性特征。两层Braarudosphaerid地层均表现出两种浅栖浮游有孔虫物种的稳定同位素信号的瞬态差异。这类瞬态δ18O偏移可能反映了这两种混合层物种在深度生态位上的细微差异。若果真如此，那么Braarudosphaerid的沉积事件可能代表了发生在透光带（euphotic zone）下部的"地下水华"。或者，这类瞬态δ18O偏移也可能反映了显著的季节性变化，即Braarudosphaerid水华发生于春季上升流时段。学界普遍认为，Braarudosphaera水华在南大西洋两侧的反复出现，反映了海洋环流涡旋（gyre circulation）的强度与格局的周期性变化。我们推测，南大西洋Braarudosphaera水华在早渐新世末期的终止，可能与德雷克海峡（Drake Passage）构造张开达到临界阈值以及南极环极流（Antarctic Circum-Polar Current）发育所引发的古海洋学变化有关。