Description: 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.

Description: Acarininid shells were cast in epoxy, ground to midsection and imaged by SEM to identify suitable domains for analysis. Fully quantitative Mg/Ca analyses by a CAMECA SX-51 Electron Probe Microanalyzer (mineral standards, background subtracted, and matrix corrected) were performed using Probe for EPMA© software. An accelerating voltage of 15 kV with 10 nA Faraday current and a tightly focused beam were used, with a counting time of 20s on the peak and 10s on each of the two background positions. These analyses were performed using the “Time Dependent Intensity” feature of the Probe for EPMA software, where Ca and Mg were measured in 4 s increments and counts were plotted against time, with extrapolation to time = 0s as the true count rate. The natural carbonate standards Delight Dolomite, Callender Calcite, and UWC-3 were used for Mg and Ca; Mg-KαX-rays were measured on two spectrometers and aggregated. Carbon was calculated within the matrix correction on an atomic basis as C = (Ca + Mg + Sr). Whenever possible, multiple Mg/Ca measurements were placed in each acarininid shell, and the average value was calculated. Data Quality Information: Only EPMA analytical totals of 98.5 – 100.5 wt.% were considered acceptable

Description: Sampling and Analyzing Method: Acarininid shells were cast in epoxy, ground to midsection and imaged by SEM to identify suitable, well-preserved domains for analysis. In situ d18O data were acquired in the WiscSIMS Laboratory at the Department of Geoscience, UW-Madison, using a CAMECA IMS-1280 large radius multicollector SIMS. (Kita et al., 2009; Valley and Kita, 2009). Depending on the size of the target, samples were analyzed with a 10 µm or 3 µm primary beam. Four consecutive measurements of UWC-3 calcite standard (d18O = 12.40 per mille Vienna standard mean ocean water; VSMOW; Kozdon et al., 2009) were performed before and after every set of typically 10 sample analyses.

Description: Micropaleontologists have traditionally recognized the mid-Miocene Fohsella lineage as a flagship for phyletic gradualism within the planktic foraminifera. However, study of a deep-sea record from the western equatorial Pacific (ODP Site 806) reveals that coiling ratios within this clade suddenly (〈5 kyr) shift after a prolonged, ancestral state of near randomness (~50%) to a transient phase (13.42–13.43 Ma) of dextral dominance (~75%) immediately following the first common occurrence of keeled fohsellids. This brief period of dextral dominance was abruptly (〈5 kyr) succeeded by an irreversible change to sinistral dominance (~96%). Fohsellid abundances decline markedly through the interval in which the sinistral preference is established. The shift to sinistrality (13.42 Ma) predated the deepening of fohsellid depth ecology by ~240–488 kyr, indicating that these two events were unrelated. This view is supported by a lack of delta 18O evidence for depth–habitat differences between the two chiral forms, which refutes the notion that sinistral fohsellids were “pre-adapted” for ensuing hydrographic change because they occupied a deeper depth habitat than their dextral counterparts. Planktic foraminiferal assemblages become strongly oligotrophic in character through the interval in which the fohsellid delta 18O increase is recorded, indicating that the migration to deeper depths was fostered by an expansion of the mixed layer in the western equatorial Pacific. Salient aspects of this brief, but conspicuous faunal change are a marked increase in the abundance of symbiont-bearing globigerinoidids, a concomitant collapse of local Jenkinsella mayeri/siakensis populations, and reduced fohsellid abundances. The rapid and permanent nature of the Fohsella sinistral shift provides a distinct, unequivocal datum that may prove useful for correlating mid-Miocene sections throughout the Caribbean Sea and tropical regions in the western sectors of the Pacific and Atlantic. The coiling ratio changes that occurred during the evolution of the Fohsella chronocline probably reflect changing population dynamics between cryptic genotypes with different coiling preferences.

Description: Rapid carbon input into the ocean-atmosphere system caused a dramatic shoaling of the lysocline during the Paleocene-Eocene thermal maximum (PETM), a transient (~170 kyr) global warming event that occurred roughly 55 Ma. Carbon cycle models invoking an accelerated carbonate-silicate feedback mechanism to neutralize ocean acidification predict that the lysocline would subsequently deepen to depths below its original position as the marine carbonate system recovered from such a perturbation. To test this hypothesis, records of carbonate sedimentation and preservation for PETM sections in the Weddell Sea (ODP Site 690) and along the Walvis Ridge depth transect (ODP Sites 1262, 1263, and 1266) were assembled within the context of a unified chronostratigraphy. The meridional gradient of undersaturation delimited by these records shows that dissolution was more severe in the subtropical South Atlantic than in the Weddell Sea during the PETM, a spatiotemporal pattern inconsistent with the view that Atlantic overturning circulation underwent a transient reversal. Deepening of the lysocline following its initial ascent is signaled by increases in %CaCO3 and coarse-fraction content at all sites. Carbonate preservation during the recovery period is appreciably better than that seen prior to carbon input with carbonate sedimentation becoming remarkably uniform over a broad spectrum of geographic and bathymetric settings. These congruent patterns of carbonate sedimentation confirm that the lysocline was suppressed below the depth it occupied prior to carbon input, and are consistent with the view that an accelerated carbonate-silicate geochemical cycle played an important role in arresting PETM conditions.