Gerta Keller | Princeton University (original) (raw)

Papers by Gerta Keller

The End-Cretaceous Mass Extinction and the Chicxulub Impact in Texas, 2011

Earth and Planetary Science Letters, 2009

Deccan intertrappean sediments in central India are generally considered as terrestrial deposits ... more Deccan intertrappean sediments in central India are generally considered as terrestrial deposits of Maastrichtian age, but the Cretaceous-Tertiary (K-T) position is still unknown. Here we report the discovery of the K-T transition, a marine incursion and environmental changes preserved within the intertrappean sediments at Jhilmili, Chhindwara District, Madhya Pradesh. Integrative biostratigraphic, sedimentologic, mineralogic and chemostratigraphic analyses reveal the basal Danian in the intertrappean sediments between lower and upper trap basalts that regionally correspond to C29r and the C29R/C29N transition, respectively. Intertrappean deposition occurred in predominantly terrestrial semi-humid to arid environments. But a short aquatic interval of fresh water ponds and lakes followed by shallow coastal marine conditions with brackish marine ostracods and early Danian zone P1a planktic foraminifera mark this interval very close to the K-T boundary. This marine incursion marks the existence of a nearby seaway, probably extending inland from the west through the Narmada and Tapti rift valleys. The Jhilmili results thus identify the K-T boundary near the end of the main phase of Deccan eruptions and indicate that a major seaway extended at least 800 km across India.

Examination of planktic foraminifera in the Tethys basin during the Paleocene-Eocene transition r... more Examination of planktic foraminifera in the Tethys basin during the Paleocene-Eocene transition reveals two stasis intervals that are separated by a major saltation event coincident with the P-E short-term perturbation in global climate and oceanography. Changes occurred at many spatial and temporal scales as well as many taxonomic and ecologic hierarchical levels, though with various rates and magnitudes. The stasis intervals are marked by slow changes at the species level and account for 50% of the observed first and last appearances during a 2.5 Myr interval. The saltation event is marked by rapid changes at the species and morpho-guild levels and accounts for the remaining 50% of first and last appearances during an interval of about 100-200 kyr. Despite these changes, many taxonomic and ecologic units, such as the depth assemblages and genera, and faunal parameters, such as species richness and turnover rates, are stable with respect to the P-E perturbation. This coexistence of change and stability marks the crisis of Tethyan planktic foraminifera across the P-E transition and reveals the possible dynamics of ecological evolution.

Lu, G. and Keller, G., 1993. The Paleocene-Eocene transition in the Antarctic Indian Ocean: Infer... more Lu, G. and Keller, G., 1993. The Paleocene-Eocene transition in the Antarctic Indian Ocean: Inference from planktic foraminifera. Mar. Micropaleontol., Isotopic depth stratification and relative abundance studies of planktic foraminifera at ODP Site 738 reveal three major faunal turnovers during the latest Paleocene and early Eocene, reflecting the climatic and structural changes in the Antarctic surface ocean.

Planktic foraminiferal analysis of sedimentary samples at 25-cm intervals (81 k.y.) at DSDP Site ... more Planktic foraminiferal analysis of sedimentary samples at 25-cm intervals (81 k.y.) at DSDP Site 577 provides a high resolution data base for deep-sea biostratigraphy and faunal changes during the late Paleocene to early Eocene. There are three major faunal turnovers, each of which occurred over a time period of 200 to 400 k.y., near the P3a/P3b, P6a/P6b and PSIP9 boundaries. These turnovers are characterized by rapid changes in species richness, large numbers of first and last appearances, and significant changes in the relative abundances of species. A chronological sequence of datum levels and faunal events of foraminifera tied to magnetostratigraphy provides biostratigraphic control at 1 m.y. intervals for the late Paleocene to early Eocene, with faunal and isotopic events near the PaleoceneiEocene boundary providing isochronous stratigraphic markers.

Marine Micropaleontology, 2004

Blooms of the disaster opportunist Guembelitria species are proxies for environmental catastrophe... more Blooms of the disaster opportunist Guembelitria species are proxies for environmental catastrophes, whether impact or volcanism, leading to severe biotic stress crises that may range from temporary exclusion of ecological specialists and generalists to mass extinctions. During the late Maastrichtian and early Danian (zones P0 and Pla), Guembelitria blooms show global distributions, but with the largest blooms (40 -80% Guembelitria) in low and middle latitudes and only minor blooms (10 -20%) in high latitudes. Late Maastrichtian Guembelitria blooms are, so far, known from the Indian Ocean and eastern Tethys. The most intense Guembelitria blooms (>60% Guembelitria) occurred in shallow continental shelf areas, slope/shelf margins and volcanic provinces of the Indian Ocean. What these environments have in common is high nutrient influx (eutrophication) either from continental runoff, upwelling along continental margins or volcanic input. At times of biotic crises, Guembelitria blooms may have spread rapidly to the exclusion of most or all other species, much like today's red tides, but with near global distributions.

Paleoceanography, 1990

ABSTRACT

Geological Society of America Bulletin, 1989

Geological Society of America Bulletin, 1991

Palaeogeography, Palaeoclimatology, Palaeoecology, 2015

ABSTRACT The Paleocene-Eocene Thermal Maximum (PETM) shows an extraordinary drop in the δ13C of c... more ABSTRACT The Paleocene-Eocene Thermal Maximum (PETM) shows an extraordinary drop in the δ13C of carbonate and organic matter across the globe, suggesting massive release of 13C-depleted carbon dioxide into the ocean and atmosphere over a very short time interval (probably b20ky). We report a geochemical and mineralogical study of 106 samples spanning the most expanded PETM at the Dababiya Global Stratotype Standard section and Point (GSSP) near Luxor, Egypt. The field and laboratory observations reveal that the deposition occurred in a submarine channel extended laterally about 200 m with the deepest part (~0.88 m) at the designated GSSP, although all bio-zones are present. Stable isotope records of both carbonate and organic carbon show decreases starting 0.6 m below the Paleocene-Eocene boundary (PEB) and culminating at the erosion surface. A persistent shift in δ15Norg values to near zero reflects a gradual increase in bacterial activity. High Ti, K and Zr and low Si contents at the PEB coincide with increased kaolinite contents, which suggests intense chemical weathering under more humid conditions at the PETM onset. Two negative Ce-anomalies indicate intervals of anoxic conditions during the lower and middle PETM (base and top of zone E1). The first anoxic event is represented by a negative Ce-anomaly, high V/C rand V/V+Ni ratios, negative Mn* and an abundance of idiomorphic pyrite crystals that indicate anoxic to euxinic conditions. The anoxic event (middle PETM) is marked by high U, Mo, V, Fe and abundant small sized (2–5 μm) pyrite framboids, increased Cu, Ni, and Cd at the samelevel suggesting anoxic conditions linked to high surface water productivity. Above this interval, oxic conditions returned as indicated by the precipitation of phosphorus and barium. These data reveal an expanded PETM interval marked by intense weathering as a crucial parameter during the recovery phase.

Cretaceous Research, 2011

Two shallow water late Cenomanian to early Turonian sequences of NE Egypt have been investigated ... more Two shallow water late Cenomanian to early Turonian sequences of NE Egypt have been investigated to evaluate the response to OAE2. Age control based on calcareous nannoplankton, planktic foraminifera and ammonite biostratigraphies integrated with δ13C stratigraphy is relatively good despite low diversity and sporadic occurrences. Planktic and benthic foraminiferal faunas are characterized by dysoxic, brackish and mesotrophic conditions, as indicated by low species diversity, low oxygen and low salinity tolerant planktic and benthic species, along with oyster-rich limestone layers. In these subtidal to inner neritic environments the OAE2 δ13C excursion appears comparable and coeval to that of open marine environments. However, in contrast to open marine environments where anoxic conditions begin after the first δ13C peak and end at or near the Cenomanian–Turonian boundary, in shallow coastal environments anoxic conditions do not appear until the early Turonian. This delay in anoxia appears to be related to the sea-level transgression that reached its maximum in the early Turonian, as observed in shallow water sections from Egypt to Morocco.► Two sequences of NE Egypt have been investigated to evaluate the response to OAE2. ► Age control based on nannoplankton, foraminifera and ammonite. ► Faunal assemblage is characterized by dysoxic, brackish and mesotrophic conditions. ► The OAE2 δ13C excursion appears comparable to that of open marine environments. ► Anoxic conditions do not appear until the early Turonian sea level transgression.

The discovery of a new Cretaceous/Paleogene (K/Pg) bathyal marine sequence on Gorgonilla Island, ... more The discovery of a new Cretaceous/Paleogene (K/Pg) bathyal marine sequence on Gorgonilla Island, SW Colombia, extends the presence of Chicxulub spherule deposits to the Pacific region of northern South America and to the Eastern Pacific Ocean. The Gorgonilla spherule layer is approximately 20 mm thick and consists of extraordinarily well-preserved glass spherules (microtektites and microkrystites) up to 1.1 mm in diameter. About 70-90 % of the spherules are vitrified, and their chemical composition is consistent with Haiti (Beloc) impact glass spherules. Normal size-grading, delicate spherule textures, welded melt components, and absence of bioturbation or traction transport indicate that the Gorgonilla spherule layer represents an almost unaltered primary suspension settling deposit.
Keywords: K/Pg boundary, Chicxulub, spherules, eastern Pacific, Colombia

Proceedings of the …, Jan 1, 2004

Since the early l990s the Chicxulub crater on Yucatan, Mexico, has been hailed as the smoking gun... more Since the early l990s the Chicxulub crater on Yucatan, Mexico, has been hailed as the smoking gun that proves the hypothesis that an asteroid killed the dinosaurs and caused the mass extinction of many other organisms at the Cretaceous-Tertiary (K-T) boundary 65 million years ago. Here, we report evidence from a previously uninvestigated core, Yaxcopoil-1, drilled within the Chicxulub crater, indicating that this impact predated the K-T boundary by Ϸ300,000 years and thus did not cause the end-Cretaceous mass extinction as commonly believed. The evidence supporting a pre-K-T age was obtained from Yaxcopoil-1 based on five independent proxies, each with characteristic signals across the K-T transition: sedimentology, biostratigraphy, magnetostratigraphy, stable isotopes, and iridium. These data are consistent with earlier evidence for a late Maastrichtian age of the microtektite deposits in northeastern Mexico. . Location of core Yax-1 and other Yucatan cores relative to seismic lines and the proposed crater size superimposed over the gravimetric anomaly map.

Marine Micropaleontology, 1997

We studied two sections that accumulated during the Paleocene±Eocene transition in shelf waters i... more We studied two sections that accumulated during the Paleocene±Eocene transition in shelf waters in the northeastern Tethys. Stable carbon isotopic compositions of marine and terrestrial biomarkers are consistent with a 13 C depletion in the oceanic and atmospheric carbon dioxide pools during the Late Paleocene Thermal Maximum (LPTM; Subzone P5b). The 2±3½ negative d 13 C excursion in planktic foraminifera coincides with minimum d 18 O values, an incursion of transient subtropical planktic foraminiferal fauna, and the occurrence of an organicrich sapropelite unit in Uzbekistan, which accumulated at the onset of a transgressive event. Biomarker distributions and hydrogen indices indicate that marine algae and bacteria were the major organic matter sources. During the Late Paleocene (Subzones P4 and P5a), the marginal northeastern Tethys experienced a temperate to warm climate with wet and arid seasons. Most likely, warm and humid climate initiated during the LPTM (Subzone P5b) and subsequently extended during the Eocene (Zone P6) onto adjacent land areas of the marginal northeastern Tethys.

Cretaceous Research, 2008

In this study we report similar biotic response patterns in planktic foraminiferal assemblages, w... more In this study we report similar biotic response patterns in planktic foraminiferal assemblages, whether in association with volcanism, impacts or climate change at the end of the Cretaceous and early Tertiary. During and after each type of catastrophe two groups dominate high stress assemblages: (1) the small Guembelitria species, which are interpreted as having thrived in eutrophic surface waters where other species rarely survived; and (2) the low oxygen tolerant small Heterohelix species, which thrived at times of an expanding oxygen minimum zone associated with high nutrients and a stratified water column.

Surface dwelling planktic foraminifera, H. planispira, register a +2.0 to 2.5 ppm delta13C excurs... more Surface dwelling planktic foraminifera, H. planispira, register a +2.0 to 2.5 ppm delta13C excursion straddling the Harland Shale and Bridge Creek Limestone member (latest Cenomanian) at the Pueblo Stratotype, Colorado. Post-excursion, the delta13C values remain at approximately 2.0 ppm dipping only slightly in the early Turonian (H. helvetica zone). During the initial phases of the delta13C excursion, 5 planktic species consecutively become extinct (Rotalipora deekei, R. greenhornensis, R. cushmani, P. inornata, G. bentonensis). These are subsequently replaced in the latter stages of the excursions by 5 new species with high depth tolerances, resulting in no net diversity loss. The extinction is therefore likely to have been as a direct result from the increased water mass stratification. A temporary decrease in bottom water productivity is also suggested by oxygenated bottom waters (presence of Cibidicoides) and a minimum of low O_2 tolerant species (hetrohelicids). Elevated HI/OH...