Sherwood W. Wise

Biogeographic gradients of middle Eocene-Oligocene calcareous nannoplankton in the South Atlantic Ocean

Middle Eocene-Oligocene calcareous nannoplankton have been analyzed quantitatively for eight DSDP and ODP sites ranging from the equatorial zone to 65°S latitude in the South Atlantic Ocean. There is a general decrease in calcareous nannoplankton species diversity from the middle Eocene through the Oligocene, and species diversity also decreases towards the higher latitudes. Similar to findings of other studies, the drop of the species diversity of calcareous nannoplankton across the Eocene/Oligocene boundary is not drastic. Delineation of latitudinal distribution patterns of individual species or groups and R-mode cluser analysis permit the classification of warm-water taxa (Coccolithus formosus, discoasters, helicosphaerids, and sphenoliths), temperate-water taxa (Coccoliths pelagicus, Cyclicargoliths floridanus group, Reticulofenestra bisecta, Reticulofenestra samodurovii/Reticulofenestra umbilica), and cool-water taxa (chiasmoliths, Isthmolithus recurvus, and Reticulofenestra daviesii). The abundance of warm-water taxa plummets whereas the abundance of cool-water taxa sharply increases across the Eocene/Oligocene boundary at the middle- and high-latitude sites, reflecting a profound cooling event in these areas. Polar ordination and similarity analyses reveal that steep latitudinal biogeographic gradients had been established at least by the middle Eocene. The gradient greatly increases near the Eocene/Oligocene boundary, indicating more severe and permanent cooling at the high latitudes. The biogeographic gradient data of this study conflict with the widely accepted inference from the oxygen isotopic data that the thermal gradients between mid latitudes and high latitudes are low or nearly flat for the Paleogene oceans. Lower surface water salinities in the high latitudes may have lowered the δ18O values of the planktonic microfossils, but apparently did not have significant effect on latitudinal distribution patterns of calcareous nannoplankton, which offers an independent means for estimating latitudinal thermal gradients.

Surface lithofacies, biofacies, and diatom diversity patterns as models for delineation of climatic change in the southeast Atlantic Ocean

Abstract Distribution of diatom species in surface sediments of the southeast Atlantic Ocean is regulated by present-day oceanographic and hydrodynamic processes. Five assemblages (vectors) defined by factor-vector analysis, reflect different environments and conditions. Assemblage A is a high diversity flora associated with the nutrient-rich, relatively cold waters south of the Polar Front in the diatom ooze belt. Occurrence of this flora on Maud Rise and vicinity may reflect periodic occurrence of a polynya in that area. Assemblage B is a reworked assemblage that dominates the southern portion of the study area. Productivity there is low, reflecting sea ice cover during most of the year. The area north of the Polar Front is dominated by Assemblage C, whose characteristic species reflect the relatively warm Subantarctic Surface Water. Winnowing and frustule breakage have altered Assemblage D (found in three isolated samples) by removing relatively delicate forms leaving lag deposits of more robust species. Assemblage E is a low diversity stress flora, reflecting unstable, unpredictable environments along the Polar Front, Antarctic slope, and the northern boundary of winter sea ice. These sites are characterized by the sinking of cold water. Downcore analysis of cores lying adjacent to the Polar Front and the diatom ooze—pelagic clay boundary show evidence for past climatic variation. The low trophic level occupied by diatoms and the subsequent sensitivity of these organisms to abiotic environmental parameters such as light quality, make relative diatom abundance a useful tool for monitoring fluctuations of winter sea ice and the temperature changes responsible for these fluctuations. The position of the Polar Front has migrated at least three times within the last 0.015 m.y. B.P. Within the last 0.3 m.y. B.P. warm maxima have occurred at approximately 0.0, 0.015, 0.125 and 0.3 m.y. B.P.

Paleogene calcareous nannofossil magnetobiochronology: Results from South Atlantic DSDP Site 516

Abstract The detailed study of an expanded Paleogene section with abundant, moderate to well preserved calcareous nannofossils from South Atlantic DSDP Site 516 has resulted in a precise correlation of most calcareous nannofossil markers with the magnetostratigraphy. Many nontraditional datums have also been documented and correlated to the magnetostratigraphy. Comparison of the results from Site 516 with those of previous studies from other areas enables a critical evaluation of the accuracy, synchroneity or diachroneity of the species events over geographically long distances. Of special significance is the correlation for the first time of the stratigraphic ranges of Chiasmolithus gigas and Rhabdosphaera gladius with the magnetostratigraphy. Other important results include the following: first occurrence (FO) of Cruciplacolithus primus , 66.3 Ma; FO Chiasmolithus danicus , 65.6–66.0 Ma; FO Prinsius martinii , 65.5–66.0 Ma; FO Heliolithus kleinpellii , 59.8–61.6 Ma (probably diachronous); last occurrence (LO) of Tribrachiatus orthostylus , 51.0–54.8 Ma (unreliable); FO Chiasmolithus gigas , 47.4 Ma; FO Reticulofenestra umbilica , 44.6 Ma; LO Chiasmolithus gigas , 44.4–46.8 Ma (diachronous); LO Nannotetrina fulgens , 44.2 Ma; LO Chiasmolithus grandis , 40.0–41.6 Ma (probably diachronous); FO Chiasmolithus oamaruensis , 39.8–40.4 Ma (unreliable); FO Isthmolithus recurvus , 39.5 Ma; LO Reticulofenestra reticulata , 37.6 Ma; LO Discoaster saipanensis , 36.4 Ma; end acme of Ericsonia subdisticha , diachronous; FO and LO Sphenolithus distentus , unreliable; and LO Reticulofenestra bisecta , 24.0 Ma.

Geologic history of the Maurice Ewing Bank of the Falkland Plateau (southwest Atlantic sector of the Southern Ocean) based on piston and drill cores

Abstract Paleontologic and sedimentologic analyses of fifty-five piston cores and three drill-core sequences, combined with seismic reflection profiler data, provide the basis for a geologic study of the intermediate-depth Maurice Ewing Bank located at the eastern end of the Falkland (Malvinas) Plateau, southwest Atlantic sector of the Southern Ocean. Presented is a geologic map and cross-section of older (pre-Pliocene) units sampled beneath a thin veneer (1–2 m) of Plio-Pleistocene siliceous ooze and glacial marine clastics which mantle the plateau. Thirty-four piston cores penetrated pre-Pliocene sediments, the oldest being Campanian in age. A comparison of faunal and floral assemblage characteristics of Cretaceous sediment from opposite sides of the Maurice Ewing Bank shows significant paleoecological differences which suggest that the Falkland Plateau served as an important barrier between water masses in the South Atlantic during this time. During the Tertiary, a relatively high carbonate compensation depth coupled with a significant variation in topography across the bank contributed to marked changes in lithologic facies ranging from coccolith ooze near the apex of the bank to diatom ooze and zeolitic clay on its flanks. Miocene coccolith ooze deposition ceased when the center of high carbonate productivity (and the Miocene Polar Front?) migrated north of the bank in response to deteriorating climatic conditions associated with a severe Late Miocene continental glaciation on the Antarctic continent. Strong bottom currents associated with the Cretaceous boundary event, the Early Miocene opening of the Drake Passage, and the Late Miocene Antarctic glaciation caused scouring and removal of significant volumes of sediment, thus contributing to the complex erosional history of the bank. During the Late Miocene the last and most severe of these events truncated the previously deposited Upper Cretaceous—Miocene sequence of sediments of the bank. The primary erosional agent during the Late Miocene was probably a northerly component of the circum-polar deep water which impinged on the bank from the southwest. Erosion of older pelagic oozes ceased with the deposition of Plio-Pleistocene glacial marine clastics which essentially “armoured” the sediment surface with a protective, erosion-resistant cover. Carbonate deposition in the study area resumed during the latest Pleistocene when the polar front migrated to a mean position near the southern margin of the bank.

Paleoenvironmental analysis and correlation of a CretaceousIslas Orcadas core from the Falkland Plateau, Southwest Atlantic

Abstract A comparison of Campanian-Maestrichtian cores from two localities on opposite sides of the Maurice Ewing Bank, at the eastern end of the east-west trending Falkland Plateau (Southwest Atlantic) reveals a significant change in faunal and floral assemblage characteristics across the Plateau. These changes are probably temperature dependent, and indicate that the Plateau served as an important barrier between water masses in the South Atlantic during late Cretaceous times.

Evolution of a deep-water carbonate platform: Upper Cretaceous to Pleistocene sedimentary environments on the west Florida margin

Abstract Deep-water carbonates have been accumulating on the west Florida margin since the early Late Cretaceous, although the depositional style and sediment types have changed through time. Seven coreholes drilled by EXXON record the establishment of two gently-dipping aggradational ramps, the first in the Late Cretaceous to Oligocene, and the second in the middle to late Miocene to the present. These dominantly pelagic depositional environments followed episodes of downslope transport of shallow-water grains, diluting background pelagic sedimentation and producing progradational ramps that built seaward. This study traces the evolution of the deep-water west Florida sedimentary systems, and contrasts the older aggradational ramp, characterized by both foraminiferal and radiolarian oozes as well as volcanic ash, with the late Neogene ramp whose biogenic components are mainly foraminifera and coccoliths. The cycles of progradation, separated by periods of ramp aggradation, developed as eustasy interacted with prolonged subsidence of the margin (early Late Cretaceous), as well as oceanographic changes caused by closure of the Suwannee Strait (late Oligocene to early Miocene) and of the Isthmus of Panama (middle to late Miocene). The major transitions in environments recorded in the west Florida ramp platform thus resulted from the superposition of climatic, tectonic, and oceanographic events on global sea level changes.