Stefan Gartner

Calcareous nannofossil biostratigraphy and revised zonation of the Pleistocene

Abstract Deep Sea Drilling Project sites 154A, 206, and 289 yielded reasonably complete Pleistocene sections of deep-sea sediments in which a good to excellent succession of calcareous nannofossils is present. A detailed study of the nannofossils from the above three sections and an evaluation of data from several piston cores have resulted in a revised zonation of the Pleistocene in which seven zones are recognized. A reasonably rigorous chronology is established for this zonal succession by correlating all possible biostratigraphic datum levels with magnetic stratigraphy in the Pacific and with paleoclimatic fluctuations in the Pacific and the Caribbean. The ages for intermediate levels are interpolated. From youngest to oldest the proposed zones for the Pleistocene and the absolute ages for their limits are as follows: Emiliania huxleyi Acme Zone 0.07 m.y. Small Gephyrocapsa Zone 1.22 to 0.92 m.y. Emiliania huxleyi Zone 0.27 to 0.07 m.y. Helicopontosphaera sellii Zone 1.51 to 1.22 m.y. Geophyrocapsa oceanica Zone 0.44 to 0.27 m.y. Cyclococcolithina macintyrei Zone 1.65 to 1.51 m.y. Pseudoemiliania lacunosa Zone 0.92 to 0.44 m.y. All of the above zones except one span time periods of 0.3 m.y. or less. The mid-Pleistocene Pseudoemiliania lacunosa Zone extends over a time interval of nearly 0.5 m.y. For the most part the marker species are cosmopolitan forms which have considerable latitudinal range, and are useful in hemipelagic as well as in oceanic Pleistocene strata.

Paleoceanography of the mid-Pleistocene

The fossil record of the Pleistocene calcareous nannoplankton indicates that during the mid-Pleistocene (0.93–1.25 my) occurred an episode of overwhelming dominance of smallGephyrocapsa. During this episode normally abundant, large size specimens of this genus (mainlyGephyrocapsa oceanica) were virtually excluded from the phytoplankton of tropical and subtropical oceans. The best modern analog of this dominantly smallGephyrocapsa assemblage is the subpolarEmiliania huxleyi assemblage, which implies that nutrient content was significantly greater and water temperature was lower in the photic water column of the tropical oceans than they are today. Increased equatorial upwelling in the oceans, on a scale much greater than today, may explain the above pattern. To achieve such broad equatorial upwelling there must be a source and a drive for cold, dense water. The Arctic Ocean, which was probably seasonally free of ice during this interval of the mid-Pleistocene, is capable of providing the requisite source as well as a drive for the inferred equatorial upwelling. The energy balance of a predominantly ice-free Arctic Ocean requires an approximately three to seven fold increase of hydrospheric heat transport from the North Atlantic to the Arctic Ocean, which dictates a corresponding or even greater increase in the volume of warm water entering the Arctic Ocean at the surface and cold dense water exiting at depth to the North Atlantic. Such enhanced dense water formation in the Arctic Ocean could drive the intensified equatorial upwelling implied by the smallGephyrocapsa dominance interval. If the above scenario is correct then the climate of the earths northern hemisphere during the mid-Pleistocene may have been very different from the younger Pleistocene climate. One manifestation of this difference may be the mid-Pleistocene shift in climatic cycle periodicity from 40 ky to 100 ky. Another important aspect is that the enhanced greenhouse effect expected during the next century because of an increase of atmospheric CO2 is thought to lead directly to melting of the Arctic Ocean ice cover and of the Greenland ice sheet. Thus, the “greenhouse” Arctic Ocean and its attendant ocean circulation would resemble the inferred mid-Pleistocene conditions.

Late Neogene paleoceanography of the eastern Caribbean, the Gulf of Mexico, and the eastern Equatorial Pacific

Abstract Calcareous nannofossil census data from the late Neogene of the Caribbean, the Gulf of Mexico, and the eastern equatorial Pacific were subjected to a multifold analysis in order to extract the paleoceanographic history of the region. A detailed nannofossil biostratigraphy is the basic time framework. The paleoenvironmental signals are deduced from fluctuations of diversity, dominance, and equitability through time; by clustering of assemblages through time and examining the composition of cluster groups; by comparison of fluctuations in carbonate content and coarse-fraction with time; and by factor analysis of the assemblage. These measures variously reflect surface water characteristics such as productivity, bottom water characteristics such as corrosiveness (dissolution), and conditions of sedimentation such as dilution of the pelagic biogenic carbonate. Diversity, dominance, and equitability which are determined by surface water characteristics, show similar trends, often in considerable detail, in the Caribbean and the Gulf of Mexico from the late Miocene through Pleistocene. Broad but definite similarities exist in these measures between the Caribbean/Gulf of Mexico and the eastern equatorial Pacific in the late Miocene to mid Pliocene, but not in the late Pliocene. Clustering groups assemblages in a broad biostratigraphic pattern corresponding approximately to a major assemblage zone. The structure and composition of cluster groups indicate strong similarity between the Caribbean and the Gulf of Mexico during the late Miocene through Pleistocene, but only weak similarity of the Caribbean and Gulf of Mexico to the eastern equatorial Pacific, and only during the early Pliocene. Surface water productivity is generally similar at the three locations into the late Pliocene but seemingly diverges in the eastern equatorial Pacific after about 2.4 m.y. Variations in dissolution intensity have generally similar trends in the late Miocene and early Pliocene in the Caribbean and the Gulf of Mexico but the trends diverge in the late Pliocene and are dissimilar in the Pleistocene. In the eastern equatorial Pacific, dissolution intensity increases with the inferred thermal subsidence of the crust. Variations in the calcium carbonate content of the sediment, essentially pelagic carbonate, follow different trends at the three sites, which is attributable to different sedimentary histories at the sites (tectonic plus eustatic sea level changes for the Caribbean; eustatic sea level changes alone for the Gulf of Mexico; and dilution by biogenic silica plus dissolution for the eastern equatorial Pacific). The various measured and derived parameters indicate that as late as the mid Pliocene - approximately 3.8 m.y. - a deep water connection existed between the Pacific and the Caribbean/Gulf of Mexico. A shallower seaway, permitting exchange of surface water between the Pacific and Gulf of Mexico, seems to have existed into the late Pliocene - to between 3.0 to 2.2 m.y., and final closure probably coincided with the sea level draw-down associated with the initial buildup of a northern hemisphere ice sheet. During the late Pliocene and early Pleistocene surface productivity oscillates markedly but seemingly with different intensities in the Caribbean and the Gulf of Mexico. The most important floral change occurs in both basins at about 0.9 m.y. when the smallGephyrocapsa dominated high-productivity nannoflora of the preceeding ⋍0.3 m.y. was replaced abruptly by a moderate-productivity flora. This turnover marks a major change in ocean circulation towards a relatively warm, stably stratified tropical surface ocean, which seems to have been affected remarkably little by some of the intense late Pleistocene high latitude climatic changes.

Late Neogene nannofossil biostratigraphy and paleoceanography of the northeastern Gulf of Mexico and adjacent areas

Abstract Two 305 m cored sections from the northwest Florida continental shelf contain a nearly complete record of late Neogene hemipelagic sedimentation. One of the sites, south and east of De Soto Canyon, is isolated from terrigenous sediment except for sediment transported in suspension. This site contains a continuous record from the late Miocene to the Recent. The second site, on the western rim of De Soto Canyon, is more expanded and continuous from the late Pliocene to the Recent. A hiatus separates the late Pliocene from the middle Miocene. Six prominent nannofossil biohorizons were recognized within the Pleistocene, seven within the Pliocene, and three within the Miocene; in addition one biohorizon marks the base of the Pleistocene and another the base of the Pliocene. Nearly all carbonate in the sediment is pelagic. Terrigenous detrital sedimentation was controlled by glacioeustatic sea level fluctuations during the Pleistocene, and sea level changes are probably responsible for fluctuations in the ratio of pelagic carbonate to clayey detritus in pre-Pleistocene sediments also. Carbonate content, coarse fraction percent, and relative abundances of environmentally sensitive nannoplankton species suggest important paleoceanographic changes in the northeastern Gulf of Mexico and adjacent areas. Fluctuations in the relative abundance of the solution-resistant coccoliths of the genus Cyclococcolithus indicate that waters at a depth of 600–1000 m were more corrosive during the late Miocene than they are today. The decrease in carbonate dissolution during the late Miocene probably was a response to gradual constriction of the Central American passage and the consequent restriction of flow of corrosive water from the Pacific. Short term fluctuations in dissolution during the Pliocene and Pleistocene are related to climatic cycles. Productivity variations in the surface waters, recorded mainly by the relative abundance of small and large morphotypes of closely related coccolith species, indicate that productivity increased during the Pliocene, but the most dramatic change — a major oceanwide increase in productivity — occurred during the Pleistocene, during and just prior to the Jaramillo magnetic event about 0.9 m.y. ago. Surprisingly the late Miocene Messinian event did not leave a significant imprint in the northeastern Gulf of Mexico.

The terminal Cretaceous event: A geologic problem with an oceanographic solution

The Danian coccolithosphores and planktonic foraminifers probably developed in an isolated and brackish Arctic Ocean during Late Cretaceous time, and when the Greenland Sea–Norwegian Sea passage opened to the North Atlantic, the Arctic Ocean water spread out as a surface layer over the world ocean. The presence of this low-salinity surface layer caused the catastrophic extinction of marine biota that has been labeled the terminal Cretaceous event.

Nannofossil diversity and equitability and fine-fraction δ13C across the Cretaceous/Tertiary boundary at Walvis Ridge Leg 74, South Atlantic

Abstract The isotopic composition and diversity of nannofossils were studied in cores from the Deep Sea Drilling Project (DSDP) Sites 525A, 527, 528, and 529 from the Walvis Ridge, South Atlantic to better understand the changes which occurred across the Cretaceous/Tertiary boundary (K/T boundary). The stratigraphic range of the samples is from theArkhangelskiella cymbiformis Zone in the Maastrichtian to theHeliolithus kleinpelli Zone in the Danian. Nannofossil diversity was high (Shannon-Weaver diversity index,H=2.5−3) in the late Cretaceous, but decreased sharply (H≈1) across the K/T boundary. The δ13C values also decrease across the K/T boundary at the four sites, suggesting a reduction in surface productivity in the South Atlantic concomitant with the reduction in diversity. During the Danian, nannofossil diversity and δ13C show some recovery approximately 500–700 k.y. after the boundary event. However, not until 2.5 Ma after the boundary event did diversity become constant. Diversity values similar to those for the late Cretaceous were not attained again in the early Paleocene interval studied. Carbon isotopic compositions similar to those from the Cretaceous were not attained until 4.5 Ma after the K/T event.

Terminal Cretaceous Extinction Scenario for a Catastrophe

All the biotic changes that occurred at the end of Cretaceous time, including the extinction of the dinosaurs, may be the result of a single terrestrial catastrophe. The Arctic spillover model, first proposed to explain the marine extinctions, would have caused a rapid and intense change in the earths climate including a lowering of temperature and of precipitation. This change in climate may have triggered a series of ecological disasters that included the radical change in the distribution of vegetation on the earth as well as the extinction of the dinosaurs.