GERTA KELLER PUBLICATIONS

Lithology, geochemistry, stable isotopes and integrated high-resolution biostratigraphy of the Berivotra and Amboanio sections provide new insights into the age, faunal turnovers, climate, sea level and environmental changes of the Maastrichtian to early Paleocene of the Mahajanga Basin of Madagascar. In the Berivotra type area, the dinosaur-rich fluvial lowland sediments of the Anembalemba Member prevailed into the earliest Maastrichtian. These are overlain by marginal marine and near-shore clastics that deepen upwards to hemipelagic middle neritic marls by 69.6 Ma, accompanied by arid to seasonally cool temperate climates through the early and late Maastrichtian. An unconformity between the Berivotra Formation and Betsiboka limestone marks the K–T boundary, and juxtaposes early Danian (zone Plc? or Pld) and latest Maastrichtian (zones CF2–CF1, Micula prinsii) sediments. Seasonally humid warm climates began near the end of the Maastrichtian and prevailed into the early Danian, accompanied by increased volcanic activity. During the late Danian (zones P1d–P2), a change to seasonally arid climates was accompanied by deepening from middle to outer neritic depths.   PDF

Climate and sea-level fluctuations across the Cretaceous-Tertiary (K-T) transition in Tunisia were examined based on bulk rock and clay mineralogies, biostratigraphy and lithology in five sections (El Melah, El Kef, Elles, Ain Settara and Seldja) spanning from open marine to shallow inner neritic environments. Late Campanian to early Danian trends examined at El Kef and Elles indicate an increasingly more humid climate associated with sea-level fluctuations and increased detrital influx that culminates at the K-T transition. This long-term trend in increasing humidity and runoff in the Tethys region is associated with middle and high latitude cooling. Results of short-term changes across the K-T transition indicate a sea-level lowstand in the latest Maastrichtian about 25^100 ka below the K-T boundary with the regression marked by increased detrital influx at El Kef and Elles and a short hiatus at Ain Settara. A rising sea-level at the end of the Maastrichtian is expressed at Elles and El Kef by deposition of a foraminiferal packstone. A flooding surface and condensed sedimentation mark the K^T boundary clay which is rich in terrestrial organic matter. The P0- P1a transition is marked by a sea-level lowstand corresponding to a short hiatus at Ain Settara where most of P0 is missing and a period of non-deposition and erosion in the lower part of P1a (64.95 Ma). At Seldja, P0 and possibly the topmost part of CF1 are missing. These sea-level fluctuations are associated with maximum humidity. These data suggest that in Tunisia, long-term environmental stresses during the last 500 ka before the K-T boundary and continuing into the early Danian are primarily related to climate and sea-level fluctuations. Within this long-term climatic trend the pronounced warm and humid event within the latest Maastrichtian Zone CF1 may be linked to greenhouse conditions induced by Deccan volcanism. The absence of any significant clay mineral variations at or near the K-T boundary and Ir anomaly suggests that the bolide impact had a relatively incidental short-term effect on climate in the Tethys region.   PDF

During the late Maastrichtian (66.8-65.5 Ma) the Asyut Basin in central Egypt experienced a breakdown of the surface to bottom gradient of the 13C/12C ratio with planktic N13C values 0.2-0.8x lighter than benthic values. Planktic foraminiferal species diversity was reduced by more than 50%, with assemblages dominated (60-90%) by the opportunistic blooms of the disaster species Guembelitria cretacea, which mimic the early Danian. The prolonged breakdown in productivity occurred during a time of tectonic activity and increased terrestrial runoff that may have resulted in highly eutrophic waters, coupled with a sea-level regression (65.5 Ma) that led to restricted circulation. Increased productivity during the short climate warming between 65.4 and 65.2 Ma is associated with increased species diversity, abundant rugoglobigerinids and common heterohelicids. At the end of the Maastrichtian, decreased productivity coincided with the K/T impact and mass extinction, followed by characteristically early Danian low diversity assemblages. The similarity of the late Maastrichtian and post-K/T impact Guembelitria-dominated assemblages reveals that the planktic foraminiferal response to the K/T catastrophe was not unique, but followed a predictable pattern of response to severe environmental perturbations.  PDF

High resolution (~5-10 kyr) planktonic foraminiferal analysis at Elles, Tunisia, reveals major changes in the structure of the Tethyan marine ecosystem during the upper Maastrichtian. During the first 1.5 Myr of the late Maastrichtian (68.3-66.8 Ma) relatively stable environmental conditions and cool temperatures are indicated by diverse planktonic foraminiferal populations with abundant intermediate and surface dwellers. A progressive cooling trend between ~66.8-65.45 Ma resulted in the decline of globotruncanid species (intermediate dwellers). This group experienced a further decline at the climax of a rapid warm event about 300 kyr before the K-T boundary. At the same time relative abundances of long ranging dominant species fluctuated considerably reflecting the high stress environmental conditions. Times of critical high stress environments during the late Maastrichtian, and particularly at the K-T boundary, are indicated by low species diversity and blooms of the opportunistic genus Guembelitria at warm-cool transition intervals. During the last 100 kyr of the Maastrichtian rapid cooling is associated with accelerated species extinctions followed by the extinction of all tropical and subtropical species at the K-T boundary.  PDF

Planktic foraminiferal faunas across the K-T transition at Elles and El Melah in northwestern and northeastern Tunisia, respectively, reveal patterns of species extinctions and species survivorship similar to those found at the El Kef stratotype and the Ain Settara sections. Slightly more than 2/3 of the species disappeared at or before the K-T boundary event and slightly less than 1/3 survived into the Danian where most disappeared sequentially within zone P1a (Parvularugoglobigerina eugubina) Relative species abundance patterns reveal that the 13-16 K-T survivors dominated (80%) the assemblages in the latest Maastrichtian, whereas the K-T extinct species were rare and totaled less than 20% of the total assemblages.

The K-T survivors are generally small with little surface ornamentation and geographically widespread from low tohigh latitudes. In contrast, K-T extinct species are large, highly ornamented and geographically restricted to low latitudes. This indicates that the K-T mass extinction was selective, rather than random, and predominantly affected the less robust tropical species. With the exception of the opportunistic Guembelitria species which dominate the early Danian, most K-T survivor species suffered severely as is evident by the decreased species populations after the K-Tevent. Their eventual demise appears to have been related to post-K-T environmental changes and competition fromevolving Tertiary species. These results reveal a complex mass extinction pattern that in addition to the K-T impactevent is keyed to long-term environmental changes preceding and following this event.  PDF

The current database indicates that the terminal decline and extinction, or near extinction, of many groups commonly attributed to an asteroid or comet impact at the Cretaceous–Tertiary (K–T) boundary (e.g., ammonites, bivalves, planktic foraminifera) began during the last 500 k:y: of the Maastrichtian. By the time of the K–T boundary, extinction-prone tropical and subtropical marine faunas and 4oras were almost gone, or had severely reduced species populations struggling to survive. The K–T boundary kill-e5ect was largely restricted to these struggling tropical and subtropical populations that accounted for 2=3 of the species among planktic foraminifera, but less than 10% of the total foraminiferal population. No signi7cant extinctions occurred among ecological generalists that dominated across latitudes. No single kill mechanism can account for this mass extinction pattern. The last 500 k:y: of the Maastrichtian were characterized by a series of rapid and extreme climate changes characterized by 3–4◦C warming between 65.4 and 65:2 Ma, major volcanic activity between 65.4 and 65:2 Ma, a spherule-producing event between 65.3 and 65:2 Ma, and an impact at the K–T boundary (65:0 Ma). All of these events caused major environmental perturbations and biotic stresses that resulted in severe reductions in species populations and extinctions that culminated at the K–T boundary. The mass extinction pattern, and the parallel environmental changes during the last 500 k:y: of the Maastrichtian, suggest that both long-term (climate, sea-level) and short-term (impact, volcanism) events contributed to the K–T boundary mass extinction.   PDF

Integrated sedimentology, mineralogy, geochemistry, and microfossil and macrofossil biostratigraphies of the Maastrichtian– early Paleocene Dakhla Formation of the Western Desert, Egypt, provide improved age resolution, information on the cyclic nature of sediment deposition, and the reconstruction of depositional environments. Age control based on integrated biostratigraphies of planktic foraminifera, calcareous nannofossils and macrofossils yields the following ages for stratigraphic and lithologic sequences. The contact between the Duwi and Dakhla formations marks the Campanian/Maastrichtian boundary (zone CF8a/b boundary) and is dated at about 71 Ma. The age of the Dakhla Formation is estimated to span from 71 Ma at the base to about 63 Ma at the top (zones CF8a–Plc). The Cretaceous/Tertiary (K/T) boundary is within the upper unit of the Kharga Shale Member and marked by a hiatus that spans from 64.5 Ma in the lower Paleocene (base Plc) to at least 65.5 Ma (base CF2, base M. prinsii zones) in the upper Maastrichtian at Gebel Gifata, the type locality of the Dakhla Formation. As a result, the Bir Abu Minqar horizon, deposited between about 64.2 and 64.5 Ma (Plc(l) zone), directly overlies the K/T boundary hiatus. Major hiatuses also span the late Maastrichtian–early Paleocene in sections to the northwest (c. 61.2–65.5 Ma at North El Qasr, c. 61.2–69 Ma at Bir Abu Minqar and c. 61.2–65.5 Ma at Farafra), and reflect increased tectonic activity.

During the Maastrichtian–early Paleocene a shallow sea covered the Western Desert of Egypt and the clastic sediment source was derived primarily from tectonic activity of the Gilf El Kebir spur to the southwest of Dakhla and the Bahariya arch. Uplift in the region resulted in major hiatuses in the late Maastrichtian–early Paleocene with increased erosion to the southwest. The area was located near the palaeoequator and experienced warm, wet, tropical to subtropical conditions characterized by low seasonality contrasts and predominantly chemical weathering (high kaolinite and smectite). A change towards perennially more humid conditions with enhanced runoff (increased kaolinite) occurred towards the end of the Maastrichtian and in the early Paleocene with shallow seas fringed by Nypa palm mangroves. Sediment deposition was predominantly cyclic, consisting of alternating sandstone/shale cycles with unfossiliferous shales deposited during sea-level highstands in inner neritic to lagoonal environments characterized by euryhaline, dysaerobic or low oxygen conditions. Fossiliferous calcareous sandstone layers were deposited in well-oxygenated shallow waters during sea-level lowstand periods.  PDF

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 13C depletion in the oceanic and atmospheric carbon dioxide pools during the Late Paleocene Thermal Maximum (LPTM; Subzone P5b). The 2–3‰ negative δ 13C excursion in planktic foraminifera coincides with minimum δ 18O values, an incursion of transient subtropical planktic foraminiferal fauna, and the occurrence of an organic-rich 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.   PDF

At El Caribe, Guatemala, we reported the presence of up to 10±15% altered glass spherules near the top of the breccia, which is located stratigraphically near the K/T boundary, and linked these to the Chicxulub impact (Stinnesbeck et al. l997). We also noted that analyses of Ir by R. Rocchia failed to show anomalous concentrations at the top of the breccia (Stinnesbeck et al. l997, p. 703). In this short note we amend this statement and elaborate as to the actual Ir values reported by Rocchia and the precise stratigraphic position of the Ir analyses made by him in our section as well as the section collected by Fourcade et al. (l998). We then place these findings within the context of new geological studies in Haiti and Guatemala.  PDF

Examination of new expanded K/T boundary sections near Beloc, Haiti, reveals deposition of a glass spherule-rich deposit (SRD) and two (PGE) anomalies (one Ir-dominated and one Pd-dominated) during the early Danian Parvularugoglobigerina eugubina Zone [Pla(l)]. The presence of the Haiti SRD within the early Danian is interpreted as being due to reworking. Ir is only slightly elevated within the SRD but forms an anomaly at the top of the SRD extending into the overlying pelagic limestones. It is unclear at present whether this Ir anomaly results from mechanical reworking of an impact at the K/T boundary, or an additional impact event in the early Danian. The second PGE anomaly upsection is dominated by Pd and Pt and is more compatible with a magmatic origin. This suggests a multi-event scenario consistent with one (and possibly two) impact(s), followed by a PGE-enriched volcanic event in the Caribbean.  PDF

Late Campanian through Maastrichtian sea-level changes are examined based on lithology, macrofossils and benthic foraminifera at the Elles and El Kef sections in Tunisia. Six major sea-level regressions are identified during the late Campanian (74.4–74.2 Ma, 74.0–72.5 Ma), the Campanian-Maastrichtian transition (72.2–70.3 Ma), early Maastrichtian (69.6–69.3 Ma, 68.9–68.3 Ma), and late Maastrichtian (∼65.5 Ma). Correlation of the Maastrichtian sea-level regressions with the oxygen isotope record of DSDP Site 525 in the middle latitude South Atlantic reveals that they coincide with episodes of high latitude cooling and appear to be of eustatic origin.   PDF

The Koshak section of the Mangyshlack Peninsula, Kazakhstan, is one of the most complete Cretaceous–Tertiary (K–T) transitions known from the boreal Paratethys. Cretaceous species richness is low (11 to 13 species), except for a peak of 20 species near the K/T boundary in the uppermost Maastrichtian (top 50 cm) that represents the temporary incursion of low-latitude taxa. This maximum species richness occurred during climatic warming associated with increased humidity, as suggested by clay mineral analyses. Biofacies analysis suggests external platform conditions at this time, followed by a more humid climate, a sea-level transgression, and deepening basinal facies in the lower Danian Subzone P1a. Shallower platform conditions resumed in Danian Subzones P1b and P1c, accompanied by a cooler and probably more arid climate. No abrupt mass extinction occurred at the Koshak K/T boundary which is marked by an Ir anomaly, a clay layer and the first appearance of Tertiary planktic foraminifera. The influx of lower-latitude species ends at or before the K/T boundary, whereas the majority of the indigenous Cretaceous assemblage survived into the Danian. These data suggest that long-term climatic changes may have been the principal factors in the progressive demise of the Cretaceous planktic foraminifera in the eastern boreal Paratethys.   PDF

Isotopie, geochemical and bulk mineralogical analyses in the Trabakua and Ermua sections. Basque Basin, reveal major changes across the Paleocene- Eocene transition. Expanded sedimentary records exhibit a gradual decrease of 1.0 %o in 5 "C values in the lower part of Zone P5 followed by a more rapid 3 %o negative excursion. The 3 %si S l3C excursion is associated with an abrupt decrease in carbonate sedimentation, increased detrital flux and decreased grain size which suggest changes in marine/atmospheric currents and/or size and structure of the ocean carbon reservoir. The clays recognized at Trabakua record a deep burial diagenesis as indicated by two generations of chlorite. the presence of mixed-layers chlorite-smectite and illite-smectite. the absence of smectite and the near absence of kaolinite. The very low 5 "O values (<-3.5%o) throughout the Trabakua and Ermua sections reflect diagenetic al¬ teration rather than paleotemperatures. Because of deep burial diagenesis and very poorly preserved microfossils. the Trabakua Pass and Ermua sections are not optimal potential stratotypes for the Paleocene-Eocene boundary.  PDF

A multidisciplinary approach to the study of a K/T boundary section on the Saharan Platform based on planktic and benthic foraminifera, calcareous nannofossils, lithology, stable isotopes, mineralogy and geochemistry reveals a biota stressed by fluctuating hyposaline, hypoxic littoral and nearshore environments, productivity changes, and a paleoclimate altering between seasonal warm to temperate and warm/humid conditions. Benthic formaminifera indicate that during the last 300 kyr of the Maastrichtian (CF1, Micula prinsii) deposition occurred in a inner neritic (littoral) environment that shallowed to a near-shore hyposaline and hypoxic environment during the last 100–200 kyr of the Maastrichtian. These conditions were accompanied by a seasonal warm to temperate climate that changed to warm/humid conditions with high rainfall, by decreasing surface productivity, and significantly decreasing planktic and benthic foraminiferal species richness. The K/T boundary is marked by an undulating erosional contact overlain by a 10 cm thick sandstone layer which is devoid of any exotic minerals or spherules. Their absence may be due to a short hiatus and the fact that the characteristic clay and red layer (zone P0) are missing. During the earliest Danian (Pla), low sea-levels prevailed with continued low oxygen, low salinity, high rainfall, high erosion and terrigenous sediment influx, accompanied by low diversity, low oxygen and low salinity tolerant species. These environmental conditions abruptly ended with erosion followed by deposition of a phosphatic siltstone layer that represents condensed sedimentation in an open (transgressive) marine environment. Above this layer, low sealevels and a return to near-shore, hyposaline and hypoxic conditions prevailed for a short interval [(base of Plc(2)] and are followed by the re-establishment of normal open marine conditions (inner neritic) comparable to the late Maastrichtian. This marine transgression is accompanied by increased productivity, and the first diversified Danian foraminiferal assemblages after the K/T boundary event and represents the return to normal biotic marine conditions. Though the K/T Seldja section represents one of the most shallow marginal sea environments studied to date for this interval, it does not represent isolated or atypical conditions. This is suggested by the similar global trends observed in sea-level fluctuations, hiatuses, as well as faunal assemblages. We conclude that on the Saharan platform of southern Tunisia, longterm environmental stresses beginning 100–200 kyr before the K/T boundary and related to climate, sea-level, nutrient, oxygen and salinity fluctuations, were the primary causes for the eventual demise of the Cretaceous fauna in the early Danian. The K/T boundary bolide impact appears to have had a relatively incidental short-term effect on this marine biota.  PDF

Limestone breccia deposits in southern Mexico, Guatemala and Belize have recently been interpreted as proximal to distal ballistic fallout deposits, generated by a bolide impact that struck Yucatan at K/T boundary time. We review the age, lithology and the depositional environment of Þve K/T boundary sections in the South Pete«n area of Guatemala (Caribe, Aserradero, Chisec, Actela, Chemal) in order to evaluate the nature and origin of K/T limestone breccia deposition. The sections are located 500 km south of the proposed impact site at Chicxulub and trend in an eastÐwest direction from the Guatemala/Mexico border to southern Belize. In four of the Þve sections examined, a breccia unit up to 50 m thick overlies reef-bearing shallow-water limestones of late Cretaceous (Campanian-Maastrichtian) age. Rhythmically bedded limestones, marls and siltstones of early Danian age overlie the breccia and were deposited under middle-to outer-neritic conditions. The breccia consists of di¤erently coloured layers of shallow-water limestones. Clast size generally decreases upsection to thin layers of predominantly rounded clasts, and these Þnegrained rudstones grade into grainstones at the top. In at least one section (EI Caribe) diagenetically altered glass spherules are present in the uppermost layers of the grainstone. These glass spherules are of strati- W. Stinnesbeck ( ) Geologisches Institut der Universita¬ t Karlsruhe, Postfach 6980, D-76128 Karlsruhe, Germany Fax:#0721 608 2138 G. Keller Department of Geosciences, Princeton University, Princeton, NJ 08544, USA J. de la Cruz á Carlos de Leo«n Ministerio de Energõ«a y Minas, Direccio«n de Hidrocarburos, Diagonal 17, 29Ð78, Zona 11, Guatemala 01011, Guatemala N. MacLeod á J. E. Whittaker Department of Paleontology, The Natural History Museum, Cromwell Road, London SW7 5BD, UK graphic position and chemical composition similar to black and yellow glass from Beloc, Haiti and Mimbral, Mexico, which some workers have chemically linked to melt glass within the breccia of the Chicxulub cores. We suggest that breccia deposition in Guatemala may have been multi-event, over an extended time period, and related to the collision of the Yucatan and Chortis plates as well as related to a major impact or volcanic event at the end of the Cretaceous.  PDF

A multidisciplinary study of a new Cretaceous-Tertiary (K/T) boundary section near Guayaquil, Ecuador, reveals an unusually cool water, low diversity planktic foraminiferal fauna and a high diversity radiolarian fauna similar to those found in southern high-latitude K/T sequences despite the fact that this section was deposited near the Cretaceous equator. The K/T boundary is located by planktic foraminifera within a narrow interval bounded by last appearances of tropical Cretaceous species and first appearances of Tertiary species including Parvularugoglobigerina eugubina. As in southern high latitudes, there is no major mass extinction of either planktic foraminifera or radiolarians at this level. A major radiolarian faunal discontinuity occurs some 6 m higher in the section within foraminiferal Zone Plc, some 300–500 kyr after the K/T event.

δ13C values from bulk carbonates show both high- and low-latitude characteristics. Similarly to low latitudes, there is a 3%. negative δ13C excursion at the K/T boundary which is generally interpreted as a major decrease in primary productivity. But unlike the low latitudes, recovery occurs within a few thousand years, as compared with 300–500 kyr, and suggests rapid nutrient influx from the Antarctic region via a current similar to the Humboldt current today. Similarly to high-latitude K/T sequences, a negative δ13C shift occurs in the early Danian Zone Plb about 300 kyr after the K/T boundary. Sedimentologic and mineralogic data indicate a late Maastrichtian with relatively low biogenic quartz and high carbonate followed by increasing biogenic quartz (>50%) and decreasing carbonate (<5%) during the early Danian. This suggests intensified atmospheric and oceanic circulation and upwelling off Ecuador during the early Danian. The K/T transition is marked by increased volcanic activity, continental erosion and terrigenous influx, but this also occurs in the early Danian Zone at the P1aP1b zonal transition and is thus not unique to the K/T boundary.

We suggest that the catastrophic biotic effects normally observed at the K/T boundary in low latitudes are greatly reduced or absent in the eastern equatorial Pacific because this region was dominated, then as now, by upwelling and current transport of nutrient-rich waters from the Antarctic Ocean. As a result, the biotic patterns are characteristic of southern high latitudes, whereas the δ13C pattern combines ameliorated low-latitude effects with predominantly high-latitude trends.   PDF