Daniela Crudeli

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.

Florisphaera profunda and the origin and diagenesis of carbonate phases in eastern Mediterranean sapropel units

High relative concentrations of the lower photic zone nannofossil Florisphaera profunda have been reported in all recent eastern Mediterranean sapropels. In the most recent sapropel (S1), high bulk sediment Sr/Ca ratios occur along with high F. profunda contents toward the base of the unit, exemplified here in four cores from 1.5–3.5 km water depth. Co-occurring biogenic carbonates contain insufficient Sr to account for these high Sr concentrations, and X-ray diffraction and selective leaching show that the high Sr/Ca ratios are due to aragonite, the CaCO3 polymorph that is rarely preserved in deep marine sediments, with ∼1 wt % Sr. The possible sources of this aragonite include (1) precipitation with surface ocean production, (2) detrital input from shallow-water sediments by high continental runoff, or (3) postdepositional diagenetic formation driven by increased pore water alkalinity resulting from sulphate reduction. The third formation mechanism for the aragonite is favored, in which case the similarity in the positions of the aragonite and F. profunda abundance maxima in sapropels is probably related to Corg accumulation and resulting sulphide diagenesis that produces high pore water alkalinity. There is clear micropaleontological evidence that dissolution of the less soluble biogenic low-Mg CaCO3 is occurring, or has occurred, during early diagenesis in these sediments despite the coexistence of the more soluble high-Mg calcite and aragonite polymorphs. Similar Sr/Ca maxima are also found associated with older sapropels, always located close both to local minima in surface ocean δ18O that signal maximum monsoon-driven runoff and to maxima in diagenetic sediment sulphide contents. High freshwater flows from monsoons are believed to drive eastern Mediterranean sapropel formation through water column stabilization that favors F. profunda production and later through development of deep water column dysoxia/anoxia because of reduced ventilation. The relative abundances of F. profunda are high between 5 and 11 14C kyr B.P. with a maximum at ∼9 14C kyr B.P. The surface ocean production changes marked by F. profunda therefore begin earlier and finish later than the formation of the S1 sapropel, which only develops between 6 and 10 14C kyr B.P.

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.

Reticulofenestra calicis n. sp., an unusual small reticulofenestrid coccolith from the Lower Pliocene of the South Caribbean Sea

A new, very small to small (2.4 to 4.6mum) reticulofenestrid coccolith, Reticulofenestra calicis n. sp., is described by scanning electron and light microscopy from the Lower Pliocene (Zone CN11 of Okada and Bukry 1980) of the South Caribbean Sea. The coccolith shows typical reticulofenestrid distal and proximal shields but the distal shield is surmounted by a cup-shaped structure formed from extensions of the inner and outer tube elements. A new genus is not introduced, although this Structure is peculiar for reticulofenestrids. We base generic attribution on the distal shield morphology. The species occasionally has few slits between the distal shield elements. Attribution to the genera Reticulofenestra instead of Pseudoemiliania is discussed in detail. R. calicis n. sp. is structurally similar to the modern Emiliania huxleyi var. corona and to Reticulofenestra maceria. R. calicis n. sp., readily recognized under the light microscope in side view, has a short stratigraphic range. If the species proves not to be under strong ecological control, it will constitute a new biostratigraphic marker for subdivision of the R. pseudoumbilicus Zone.