Silke Steph

Assessing the effect of dissolution on planktonic foraminiferal Mg/Ca ratios: Evidence from Caribbean core tops

In order to assess the dissolution effect on foraminiferal Mg/Ca ratios, we analyzed Mg/Ca of seven planktonic foraminiferal species and four of their varieties from Caribbean core tops from ∼900–4700 m water depth. Depending on the foraminiferal species and variety, Mg/Ca start to decline linearly below Δ[CO32−] levels of ∼18–26 μmol/kg by ∼0.04–0.11 mmol/mol per 1 μmol/kg Δ[CO32−], similar to decreases of ∼0.5–0.8 mmol/mol per kilometer below ∼2500–3000 m water depth. Above these species-specific critical levels, Mg/Ca remains stable with higher intraspecific Mg/Ca variability than below. We developed routines to correct Mg/Ca from below these critical thresholds for dissolution effects, which reduce the overall intraspecific variability by ∼24–64%, and provide dissolution-corrected Mg/Ca appropriate to calculate Holocene paleotemperatures. When taking into account only dissolution-unaffected Mg/Ca from <2000 m, the systematic succession of foraminiferal species according to their Mg/Ca reflects expected calcification depths.

Early Pliocene increase in thermohaline overturning : a precondition for the development of the modern equatorial Pacific cold tongue

Unraveling the processes responsible for Earth’s climate transition from an “El Nino–like state” during the warm early Pliocene into a modern‐like “La Nina–dominated state” currently challenges the scientific community. Recently, the Pliocene climate switch has been linked to oceanic thermocline shoaling at ∼3 million years ago along with Earth’s final transition into a bipolar icehouse world. Here we present Pliocene proxy data and climate model results, which suggest an earlier timing of the Pliocene climate switch and a different chain of forcing mechanisms. We show that the increase in North Atlantic meridional overturning circulation between 4.8 and 4.0 million years ago, initiated by the progressive closure of the Central American Seaway, triggered overall shoaling of the tropical thermocline. This preconditioned the turnaround from a warm eastern equatorial Pacific to the modern equatorial cold tongue state about 1 million years earlier than previously assumed. Since ∼3.6–3.5 million years ago, the intensification of Northern Hemisphere glaciation resulted in a strengthening of the trade winds, thereby amplifying upwelling and biogenic productivity at low latitudes.

Changes in Caribbean surface hydrography during the Pliocene shoaling of the Central American Seaway

Pliocene d18O records of shallow and deep dwelling planktonic foraminifers from the Caribbean (Ocean Drilling Program sites 999 and 1000), the tropical east Pacific (sites 1241 and 851), and the Atlantic (site 925, Ceara Rise, and site 1006, western Great Bahama Bank) were used to examine Atlantic-Caribbean-Pacific atmospheric and oceanic linkages associated with the progressive closure of the Central American Seaway (5.5–3 Ma). Comparisons indicate the development of an inner-Caribbean salinity gradient in the mixed layer and salinity changes on precessional periodicities after 4.4 Ma (site 1000), when the Pacific-Caribbean throughflow became significantly restricted. The high-amplitude variability in salinity is also observed at site 1006, monitoring the Caribbean outflow into the Atlantic. Comparisons of Caribbean and Atlantic planktonic d18O records suggest the North Atlantic subtropical gyre as a major source for high-salinity surface waters. Precession-induced variations in the volume transport of Pacific surface water masses through the Panamanian Seaway are considered a main factor to explain the Caribbean salinity minima. Results from a coupled climate model point to changes in the El Nin˜o–Southern Oscillation state as a potential trigger for changes in the amount of Pacific inflow into the Caribbean.

Foraminiferal Mg/Ca increase in the Caribbean during the Pliocene: Western Atlantic Warm Pool formation, salinity influence, or diagenetic overprint?

We constructed a high-resolution Mg/Ca record on the planktonic foraminifer Globigerinoides sacculifer in order to explore the change in sea surface temperature (SST) due to the shoaling of the Isthmus of Panama as well as the impact of secondary factors like diagenesis and large salinity fluctuations. The study covers the latest Miocene and the early Pliocene (5.6–3.9 Ma) and was combined with δ18O to isolate changes in sea surface salinity (SSS). Before 4.5 Ma, SSTMg/Ca and SSS show moderate fluctuations, indicating a free exchange of surface ocean water masses between the Pacific and the Atlantic. The increase in δ18O after 4.5 Ma represents increasing salinities in the Caribbean due to the progressive closure of the Panamanian Gateway. The increase in Mg/Ca toward values of maximum 7 mmol/mol suggests that secondary influences have played a significant role. Evidence of crystalline overgrowths on the foraminiferal tests in correlation with aragonite, Sr/Ca, and productivity cyclicities indicates a diagenetic overprint on the foraminiferal tests. Laser ablation inductively coupled plasma–mass spectrometry analyses, however, do not show significantly increased Mg/Ca ratios in the crystalline overgrowths, and neither do calculations based on pore water data conclusively result in significantly elevated Mg/Ca ratios in the crystalline overgrowths. Alternatively, the elevated Mg/Ca ratios might have been caused by salinity as the δ18O record of Site 1000 has been interpreted to represent large fluctuations in SSS, and cultivating experiments have shown an increase in Mg/Ca with increasing salinity. We conclude that the Mg/Ca record <4.5 Ma can only reliably be considered for paleoceanographical purposes when the minimum values, not showing any evidence of secondary influences, are used, resulting in a warming of central Caribbean surface water masses after 4.5 Ma of ∼2°C.

Bipolar seesaw in the northeastern tropical Atlantic during Heinrich stadials

Two SST records based on Mg/Ca of G. ruber (pink) from the continental slope off West Africa at 15°N and 12°N shed new light on the thermal bipolar seesaw pattern in the northeastern tropical Atlantic during periods of reduced Atlantic Meridional Overturning Circulation (AMOC) associated with Heinrich stadials H1 to H6. The two records indicate that the latitudinal position of the bipolar seesaws zero-anomaly line, between cooling in the North and warming in the South, gradually shifted southward from H6 to H1. A conceptual model is presented that aims to provide a physically consistent mechanism for the southward migration of the seesaws fulcrum. The conceptual model suggests latitudinal movements of the Intertropical Convergence Zone, driven by a combination of orbital-forced changes in the meridional temperature gradient within the realm of the Hadley cell and the expansion of the Northern Hemisphere cryosphere, as a major factor.

Pushing the boundaries: Glacial/interglacial variability of intermediate and deep waters in the southwest Pacific over the last 350,000 years

Glacial/interglacial changes in Southern Oceans air-sea gas exchange have been considered as important mechanisms contributing to the glacial/interglacial variability in atmospheric CO2. Hence, understanding past variability in Southern Ocean intermediate to deep water chemistry and circulation is fundamental to constrain the role of these processes on modulating glacial/interglacial changes in the global carbon cycle. Our study focused on the glacial/interglacial variability in the vertical extent of southwest Pacific Antarctic Intermediate Water (AAIW). We compared carbon and oxygen isotope records from epibenthic foraminifera of sediment cores bathed in modern AAIW and Upper Circumpolar Deep Water (UCDW; 943-2066 m water depth) to monitor changes in water mass circulation spanning the past 350,000 years. We propose that pronounced freshwater input by melting sea ice into the glacial AAIW significantly hampered the downward expansion of southwest Pacific AAIW, consistent with climate model results for the Last Glacial Maximum. This process led to a pronounced upward displacement of the AAIW-UCDW interface during colder climate conditions and therefore to an expansion of the glacial carbon pool.

Sub-Milankovitch cycles in periplatform carbonates from the early Pliocene Great Bahama Bank

High-resolution bulk sediment (magnetic susceptibility and aragonite content) and δ18O records from two different planktonic foraminifera species were analyzed in an early Pliocene core interval from the Straits of Florida (Ocean Drilling Program site 1006). The δ18O record of the shallow-dwelling foraminifera G. sacculifer and the aragonite content are dominated by sub-Milankovitch variability. In contrast, magnetic susceptibility and the δ18O record of the deeper-dwelling foraminifera G. menardii show precession cycles. The relationship between the aragonite and the paleoproxy data suggests that the export of sediment from the adjacent Great Bahama Bank was triggered directly by atmospheric processes rather than by sea level change. We propose a climate mechanism that bears similarities with the semiannual cycle component of eastern equatorial Pacific sea surface temperatures under present-day conditions.

Global Impact of the Panamanian Seaway Closure

Closure of the Isthmus of Panama about 3 million years ago (Ma) was accompanied by dramatic changes in Earths climate and biosphere. The Greenland ice sheet grew to continental extent and the great cycles of ice ages commenced dominating climate variability henceforth. Disruption of water mass exchange between the Atlantic and the Pacific Oceans led to different evolution of marine species on either side of the land bridge, while land-based organisms including mammals and other animals took the advantage to colonize an entire subcontinent. A 2-day workshop at the University of Kiel (Germany) summarized our current knowledge of this time period and identified areas for new research.

Hydroclimatic Variability in the Panama Bight Region During Termination 1 and the Holocene

A transect of sediment cores from high-sedimentation rate locations from the Panama Bight (eastern tropical Pacific) in combination with climate model experiments provides an opportunity to improve our understanding of the role of the tropical hydrologic cycle as a potential driver of global climate change during the Holocene and Termination 1. The reconstruction of regional sea-surface salinity patterns suggests the development of an anomalous precipitation dipole in the tropical eastern Pacific during Heinrich Stadial 1 (H1) with reduced rainfall over the western Panama Basin and off Costa Rica, and wetter conditions along the Colombian coast. Freshwater hosing experiments with the climate model CCSM3, mimicking the climatic reorganizations during H1, capture this precipitation dipole, while showing no change in the Atlantic-to-Pacific water vapor flux in response to a slowdown of the Atlantic meridional overturning circulation (AMOC). We conclude that the cross-isthmus vapor flux feedback on AMOC variations is negligible.