During the warm early Pliocene (~4.5 to 3.0 million years ago), the most recent interval with a climate warmer than today, the eastern Pacific thermocline was deep and the average west-to-east sea ...surface temperature difference across the equatorial Pacific was only 1.5 ± 0.9°C, much like it is during a modern El Niño event. Thus, the modern strong sea surface temperature gradient across the equatorial Pacific is not a stable and permanent feature. Sustained El Niño-like conditions, including relatively weak zonal atmospheric (Walker) circulation, could be a consequence of, and play an important role in determining, global warmth.
Earth's mightiest ocean current, the Antarctic Circumpolar Current (ACC), regulates the exchange of heat and carbon between the ocean and the atmosphere, and influences vertical ocean structure, ...deep-water production and the global distribution of nutrients and chemical tracers. The eastward-flowing ACC occupies a unique circumglobal pathway in the Southern Ocean that was enabled by the tectonic opening of key oceanic gateways during the break-up of Gondwana (for example, by the opening of the Tasmanian Gateway, which connects the Indian and Pacific oceans). Although the ACC is a key component of Earth's present and past climate system, the timing of the appearance of diagnostic features of the ACC (for example, low zonal gradients in water-mass tracer fields) is poorly known and represents a fundamental gap in our understanding of Earth history. Here we show, using geophysically determined positions of continent-ocean boundaries, that the deep Tasmanian Gateway opened 33.5 ± 1.5 million years ago (the errors indicate uncertainty in the boundary positions). Following this opening, sediments from Indian and Pacific cores recorded Pacific-type neodymium isotope ratios, revealing deep westward flow equivalent to the present-day Antarctic Slope Current. We observe onset of the ACC at around 30 million years ago, when Southern Ocean neodymium isotopes record a permanent shift to modern Indian-Atlantic ratios. Our reconstructions of ocean circulation show that massive reorganization and homogenization of Southern Ocean water masses coincided with migration of the northern margin of the Tasmanian Gateway into the mid-latitude westerly wind band, which we reconstruct at 64° S, near to the northern margin. Onset of the ACC about 30 million years ago coincided with major changes in global ocean circulation and probably contributed to the lower atmospheric carbon dioxide levels that appear after this time.
The Paleocene-Eocene Thermal Maximum (PETM) has been attributed to a rapid rise in greenhouse gas levels. If so, warming should have occurred at all latitudes, although amplified toward the poles. ...Existing records reveal an increase in high-latitude sea surface temperatures (SSTs) ($8\textdegree to 10\textdegree C$) and in bottom water temperatures ($4\textdegree to 5\textdegree C$). To date, however, the character of the tropical SST response during this event remains unconstrained. Here we address this deficiency by using paired oxygen isotope and minor element (magnesium/calcium) ratios of planktonic foraminifera from a tropical Pacific core to estimate changes in SST. Using mixed-layer foraminifera, we found that the combined proxies imply a$4\textdegree to 5\textdegree C$rise in Pacific SST during the PETM. These results would necessitate a rise in atmospheric pCO2to levels three to four times as high as those estimated for the late Paleocene.
Large amounts of 13C‐depleted carbon were released to the oceans and atmosphere during a period of abrupt global warming at the Paleocene‐Eocene thermal maximum (PETM) (∼55 Ma). Investigations of ...qualitative sedimentologic and paleontologic redox proxies such as bioturbation and benthic assemblages from pelagic and hemipelagic sections suggest transient reductions in bottom water oxygen during this interval, possibly on a global scale. Here, we present bulk sediment manganese (Mn) and uranium (U) enrichment factors (EF) in Atlantic and Pacific deep‐sea cores to constrain relative paleoredox changes across the PETM. Mn EF range from 1 to 9 in Atlantic sites, 1 to 35 in Southern Ocean sites, and are at crustal averages (EF = 1) in Pacific sites. U EF range from 1 to 5 in Atlantic sites, 1 to 90 in Southern Ocean sites, and are at crustal averages in Pacific sites. Our results indicate suboxic conditions prior to, during, and in the recovery from the PETM at intermediate depth sites in the Atlantic and Southern Ocean while the Pacific sites remained relatively oxygenated. The difference in oxygenation between the Atlantic and Pacific sites leads us to suggest the source for isotopically light carbon release during the PETM was in the Atlantic.
Key Points
We measure concentrations of Mn and U in marine PETM sediments
North Atlantic bottom waters were suboxic relative to the Pacific during the PETM
The source of methane release during the PETM was likely in the North Atlantic
We report an optimized method for extracting neodymium (Nd) from fossil fish teeth with a single-stage column (125
µl stem volume; LN Resin, Eichrom Industries, Darien Illinois) for isotopic analysis ...by multi-collector inductively coupled mass spectrometry (MC-ICMPS). Three reference materials (basalt: BCR-2, BHVO-2; phosphate: fossil bone composite) and splits of fossil fish teeth samples previously processed with existing two-stage column methods were processed using the single-stage column method.
143Nd/
144Nd values of reference materials agree within error with published values, and the values for fish teeth correspond with sample splits processed with two-stage columns. Precision to ±
∼
0.23
ε
Nd was achieved for 30
ng Nd samples of reference materials, and Nd isotope measurements of fossil fish tooth sample replicates as small as 7
ng Nd were reproducible within long term instrumental uncertainty. We demonstrate the utility of the new method with the first high resolution Nd isotope record spanning the ∼
40.0
Ma middle Eocene Climatic Optimum, which shows an excursion of 0.65
ε
Nd during the peak warming at the study site (Ocean Drilling Program Leg 119, Site 738; 30
kyr sample spacing from 40.3 to 39.6
Ma). LN Resin is already used in standard methods for separating Nd, and Nd isotopes are routinely measured by MC-ICPMS with high efficiency inlet systems. Our innovation is a single, small volume LN Resin column for Nd separation. The streamlined approach results in a 10× increase in sample throughput.
Constraining the magnitude of high-latitude temperature change across the Eocene–Oligocene transition (EOT) is essential for quantifying the magnitude of Antarctic ice-sheet expansion and ...understanding regional climate response to this event. To this end, we constructed high-resolution stable oxygen isotope (δ18O) and magnesium/calcium (Mg/Ca) records from planktic and benthic foraminifera at four Ocean Drilling Program (ODP) sites in the Southern Ocean. Planktic foraminiferal Mg/Ca records from the Kerguelen Plateau (ODP Sites 738, 744, and 748) show a consistent pattern of temperature change, indicating 2–3°C cooling in direct conjunction with the first step of a two-step increase in benthic and planktic foraminiferal δ18O values across the EOT. In contrast, benthic Mg/Ca records from Maud Rise (ODP Site 689) and the Kerguelen Plateau (ODP Site 748) do not exhibit significant temperature change. The contrasting temperature histories derived from the planktic and benthic Mg/Ca records are not reconcilable, since vertical δ18O gradients remained nearly constant at all sites between 35.0 and 32.5Ma. Based on the coherency of the planktic Mg/Ca records from the Kerguelen Plateau sites and complications with benthic Mg/Ca paleothermometry at low temperatures, the planktic Mg/Ca records are deemed the most reliable measure of Southern Ocean temperature change. We therefore interpret a uniform cooling of 2–3°C in both deep surface (thermocline) waters and intermediate deep waters of the Southern Ocean across the EOT. Cooling of Southern Ocean surface waters across the EOT was likely propagated to the deep ocean, since deep waters were primarily sourced on the Antarctic margin throughout this time interval. Removal of the temperature component from the observed foraminiferal δ18O shift indicates that seawater δ18O values increased by 0.6±0.15‰ across the EOT interval, corresponding to an increase in global ice volume to a level equivalent with 60–130% modern East Antarctic ice sheet volume.
► Southern Ocean temperature history across Eocene–Oligocene transition. ► Study interval spanning 35.0 to 32.5Ma. ► High-resolution foraminiferal Mg/Ca and δ18O records constructed at four study sites. ► 2–3°C cooling of Southern Ocean surface waters across Eocene–Oligocene transition. ► Moderate deep-water cooling and full-scale Antarctic glaciation interpreted at 34Ma.
Sepsis is an emerging life-threatening entity and a worldwide epidemic. Nurses are in key positions to identify patients with sepsis, mobilize the medical team, and implement interventions. A study ...of self-assessed nurse competence was conducted to determine the influence of a specially designed sepsis education program on nurses' perceived ability to identify early, intervene, and care for patients with sepsis. The program was a multimodal design incorporating online interactive didactic presentations, video vignettes, pre- and postknowledge tests, and high-fidelity medical simulation scenarios. A sample of 82 critical care and emergency department nurses in a 1-year critical care nurse training program was used for this study. Pretest and posttest module knowledge scores and self-assessed competence data were collected and analyzed. No improvement in the overall self-assessed competence scores was found; however, self-perceived frequency of use of competence behaviors improved. Participants felt more competent on three sepsis-targeted statements, and posttest knowledge scores showed significant improvement.
An understanding of sediment redox conditions across the Paleocene‐Eocene thermal maximum (PETM) (∼55 Ma) is essential for evaluating changes in processes that control deep‐sea oxygenation, as well ...as identifying the mechanisms responsible for driving the benthic foraminifera extinction. Sites cored on the flanks of Walvis Ridge (Ocean Drilling Program Leg 208, Sites 1262, 1266, and 1263) allow us to examine changes in bottom and pore water redox conditions across a ∼2 km depth transect of deep‐sea sediments of PETM age recovered from the South Atlantic. Here we present measurements of the concentrations of redox‐sensitive trace metals manganese (Mn) and uranium (U) in bulk sediment as proxies for redox chemistry at the sediment‐water interface and below. All three Walvis Ridge sites exhibit bulk Mn enrichment factors (EF) ranging between 4 and 12 prior to the warming, values at crustal averages (Mn EF = 1) during the warming interval, and a return to pre‐event values during the recovery period. U enrichment factors across the PETM remains at crustal averages (U EF = 1) at Site 1262 (deep) and Site 1266 (intermediate depth). U enrichment factors at Site 1263 (shallow) peaked at 5 immediately prior to the PETM and dropped to values near crustal averages during and after the event. All sites were lower in dissolved oxygen content during the PETM. Before and after the PETM, the deep and intermediate sites were oxygenated, while the shallow site was suboxic. Our geochemical results indicate that oxygen concentrations did indeed drop during the PETM but not sufficiently to cause massive extinction of benthic foraminifera.
Despite the recognition of the importance of phosphorus (P) in regulating marine productivity in some modern oceanic systems and over long timescales, the nature of particulate P within the ocean is ...not well understood. We analyzed P concentration in particulate matter from sediment traps and selected core tops from a wide range of oceanic regimes: open ocean environments (Equatorial Pacific, North Central Pacific), polar environments (Ross Sea, Palmer Deep), and coastal environments (Northern California Coast, Monterey Bay, Point Conception). These sites represent a range of productivity levels, temporal (seasonal to annual) distributions, and trap depths (200–4400 m). P associations were identified using an operationally defined sequential extraction procedure. We found that P in the sediment traps is typically composed of reactive P components including acid-insoluble organic P (∼
40%), authigenic P (∼
25%), and oxide associated and/or labile P (∼
21%), with lesser proportions of non-reactive detrital P depending on location (∼
13%). The concentrations and fluxes of all particulate P components except detrital P decrease or remain constant with depth between the shallowest and the deepest sediment traps, indicating some regeneration of reactive P components. Transformation from more labile forms of P to authigenic P is evident between the deepest traps and core top sediments. Although for most sites the magnitudes of reactive P fluxes are seasonally variable and productivity dependent, the fractional associations of reactive P are independent of season. We conclude that P is transported from the upper water column to the sediments in various forms previously considered unimportant. Thus, acid-insoluble organic P measurements (typically reported as particulate organic P) likely underestimate biologically related particulate P, because they do not include the labile, oxide-associated, or authigenic P fractions that often are or recently were biologically related. Organic C to reactive P ratios are typically higher than Redfield Ratio and are relatively constant with depth below ∼
300 m suggesting that preferential regeneration of P relative to C occurs predominantly at shallow depths in the water column, but not deeper in the water column (>
300 m). The view of P cycling in the oceans should be revised (1) to include P fractions other than acid-soluble organic P as important carriers of reactive P in rapidly sinking particles, (2) to include the efficient transformation of labile forms of P to authigenic P in the water column as well as in sediments, and (3) to consider the occurrence of preferential P regeneration at very shallow depths.
The Middle Eocene Climatic Optimum (MECO) is an enigmatic warming event that represents an abrupt reversal in long‐term cooling through the Eocene. In order to further assess the timing and nature of ...this event, we have assembled stable isotope and calcium carbonate concentration records from multiple Deep Sea Drilling Project and Ocean Drilling Program sites for the time interval between ∼43 and 38 Ma. Revised stratigraphy at several sites and compilation of δ18O records place peak warming during the MECO event at 40.0 Ma (Chron C18n.2n). The identification of the δ18O excursion at sites in different geographic regions indicates that the climatic effects of this event were globally extensive. The total duration of the MECO event is estimated at ∼500 ka, with peak warming lasting <100 ka. Assuming minimal glaciation in the late middle Eocene, ∼4°–6°C total warming of both surface and deep waters is estimated during the MECO at the study sites. The interval of peak warming at ∼40.0 Ma also coincided with a worldwide decline in carbonate accumulation at sites below 3000 m depth, reflecting a temporary shoaling of the calcite compensation depth. The synchroneity of deep‐water acidification and globally extensive warming makes a persuasive argument that the MECO event was linked to a transient increase in atmospheric pCO2. The results of this study confirm previous reports of significant climatic instability during the middle Eocene. Furthermore, the direct link between warming and changes in the carbonate chemistry of the deep ocean provides strong evidence that changes in greenhouse gas concentrations exerted a primary control on short‐term climate variability during this critical period of Eocene climate evolution.
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