Foraminiferal oxygen and carbon isotope data for DSDP Sites 32-305 and 62-463 and ODP Sites 171-1049 and 171-1050

Oxygen isotope analyses of well-preserved foraminifera from Blake Nose (30°N paleolatitude, North Atlantic) and globally distributed deep-sea sites provide a long-term paleotemperature record for the late Albian-Maastrichtian interval that is difficult to reconcile with the existence of significant Cretaceous ice sheets. Given reasonable assumptions about the isotopic composition of Cretaceous seawater, our results suggest that middle bathyal water temperatures at Blake Nose increased from ~12°C in the late Albian through middle Cenomanian to a maximum of 20°C during the latest Cenomanian and earliest Turonian. Bottom waters were again ~12°C during the middle Campanian and cooled to a minimum of 9°C during the Maastrichtian. Correlative middle bathyal foraminifera from other ocean basins yield paleotemperature estimates that are very similar to those from Blake Nose. Comparison of global bottom-water temperatures and latitudinal thermal gradients suggests that global climate changed from a warm greenhouse state during the late Albian through late Cenomanian to a hot greenhouse phase during the latest Cenomanian through early Campanian, then to cool greenhouse conditions during the mid-Campanian through Maastrichtian.

Chert intervals in DSDP and ODP sites (Table 1)

Radiolarian cherts in the Tethyan realm of Jurassic age were recently interpreted as resulting from high biosiliceous productivity along upwelling zones in subequatorial paleolatitudes the locations of which were confirmed by revised paleomagnetic estimates. However, the widespread occurrence of cherts in the Eocene suggests that cherts may not always be reliable proxies of latitude and upwelling zones. In a new survey of the global spatio-temporal distribution of Cenozoic cherts in Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP) sediment cores, we found that cherts occur most frequently in the Paleocene and early Eocene, with a peak in occurrences at ~50 Ma that is coincident with the time of highest bottom water temperatures of the early Eocene climatic optimum (EECO) when the global ocean was presumably characterized by reduced upwelling efficiency and biosiliceous productivity. Cherts occur less commonly during the subsequent Eocene global cooling trend. Primary paleoclimatic factors rather than secondary diagenetic processes seem therefore to control chert formation. This timing of peak Eocene chert occurrence, which is supported by detailed stratigraphic correlations, contradicts currently accepted models that involve an initial loading of large amounts of dissolved silica from enhanced weathering and/or volcanism in a supposedly sluggish ocean of the EECO, followed during the subsequent middle Eocene global cooling by more vigorous oceanic circulation and consequent upwelling that made this silica reservoir available for enhanced biosilicification, with the formation of chert as a result of biosilica transformation during diagenesis. Instead, we suggest that basin-basin fractionation by deep-sea circulation could have raised the concentration of EECO dissolved silica especially in the North Atlantic, where an alternative mode of silica burial involving widespread direct precipitation and/or absorption of silica by clay minerals could have been operative in order to maintain balance between silica input and output during the upwelling-deficient conditions of the EECO. Cherts may therefore not always be proxies of biosiliceous productivity associated with latitudinally focused upwelling zones.

The evolutionary history of size variation of planktic foraminiferal assemblages in the Cenozoic

The iterative evolutionary radiation of planktic foraminifers is a well-documented macroevolutionary process. Here we document the accompanying size changes in entire planktic foraminiferal assemblages for the past 70 My and their relationship to paleoenvironmental changes. After the size decrease at the Cretaceous/Paleogene (K/P) boundary, high latitude assemblages remained consistently small. Size evolution in low latitudes can be divided into three major phases: the first is characterized by dwarfs (65-42 Ma), the second shows moderate size fluctuations (42-14 Ma), and in the third phase, planktic foraminifers have grown to the unprecedented sizes observed today. Our analyses of size variability with paleoproxy records indicate that periods of size increase coincided with phases of global cooling (Eocene and Neogene). These periods were characterized by enhanced latitudinal and vertical temperature gradients in the oceans and high diversity (polytaxy). In the Paleocene and during the Oligocene, the observed (minor) size changes of the largely low-diversity (oligotaxic) assemblages seem to correlate with productivity changes. However, polytaxy per se was not responsible for larger test sizes.

(Table T1) Correlations between marker beds and drill holes for ODP Leg 171B sites

More than 50 discrete volcanic ash layers were recovered at the five drill sites of the Blake Nose depth transect (Leg 171B, western central Atlantic). The majority of these ash layers are intercalated with Eocene hemipelagic sediments with a pronounced frequency maximum in the upper Eocene. Several ash layers appear to be deposited from volcanic fallout with little or no indication of secondary remobilization. They provide excellent stratigraphic markers for a correlation of the Leg 171B drill sites. Other ash layers were probably redeposited from volcaniclastic-rich turbidity currents, but they still represent geologically instantaneous events that can be used in stratigraphic correlation between adjacent drill holes. Additional nonvolcanic marker beds, like the suspect late Eocene impact event layer, were included in our hole-to-hole correlations. Stratigraphic and downcore positions of marker beds were compiled and plotted against existing composite depth records that were constructed to guide high-resolution sampling. Comparison of our correlation with the spliced composite sections of each drill site reveals several minor and some major discrepancies. These may result from drilling distortion or missing sections, from the lack of unambiguous criteria for the synchronism of ash layers, or from the systematic exclusion of marker-bed data in the construction of the spliced record. Integration of both correlation approaches will help eliminate most of the observed discrepancies.

Isotopic record across the Aptian/Albian boundary of ODP Hole 171-1049C

Geochemical analyses of extraordinarily well preserved late Aptian-early Albian foraminifera from Blake Nose (Ocean Drilling Program Site 1049) reveal rapid shifts of d18O, d13C, and 87Sr/88Sr in the subtropical North Atlantic that may be linked to a major planktic foraminifer extinction event across the Aptian/Albian boundary. The abruptness of the observed geochemical shifts and their coincidence with a sharp lithologic contact is explained as an artifact of a previously undetected hiatus of 0.8-1.4 million years at the boundary contact, but the values before and after the hiatus indicate that major oceanographic changes occurred at this time. 87Sr/88Sr increase by ~0.000200, d13C values decrease by 1.5 per mil to 2.2 per mil, and d18O values decrease by ~1.0 per mil (planktics) to 0.5 per mil (benthics) across the hiatus. Further, both 87Sr/88Sr ratios and d18O values during the Albian are anomalously high. The 87Sr/88Sr values deviate from known patterns to such a degree that an explanation requires either the presence of inter-basin differences in seawater 87Sr/88Sr during the Albian or revision of the seawater curve. For d18O, planktic values in some Aptian samples likely reflect a diagenetic overprint, but preservation is excellent in the rest of the section. In well preserved material, benthic foraminiferal values are largely between 0.5 and 0.0 per mil and planktic samples are largely between 0.0 per mil to -1.0 per mil, with a brief excursion to -2.0 per mil during OAE 1b. Using standard assumptions for Cretaceous isotopic paleotemperature calculations, the d18O values suggest bottom water temperatures (at ~1000 -1500 m) of 8-10°C and surface temperatures of 10-14°C, which are 4-6°C and 10-16°C cooler, respectively, than present-day conditions at the same latitude. The cool subtropical sea surface temperature estimates are especially problematic because other paleoclimate proxy data for the mid-Cretaceous and climate model predictions suggest that subtropical sea surface temperatures should have been the same as or warmer than at present. Because of their exquisite preservation, whole scale alteration of the analyzed foraminifera is an untenable explanation. Our proposed solution is a high evaporative fractionation factor in the early Albian North Atlantic that resulted in surface waters with higher d18O values at elevated salinities than commonly cited in Cretaceous studies. A high fractionation factor is consistent with high rates of vapor export and a vigorous hydrological cycle and, like the Sr isotopes, implies limited connectivity among the individual basins of the Early Cretaceous proto-Atlantic ocean.

Determination of sulfite and manganese in sediments from the K/T boundary

The solid-state-physics technique of electron spin resonance (ESR) has been employed in an exploratory study of marine limestones and impact-related deposits from Cretaceous-Tertiary (KT) boundary sites including Spain (Sopelana and Caravaca), New Jersey (Bass River), the U.S. Atlantic continental margin (Blake Nose, ODP Leg 171B/1049/A), and several locations in Belize and southern Mexico within -600 km of the Chicxulub crater. The ESR spectra of SO3(1-) (a radiation-induced point defect involving a sulfite ion substitutional for CO3(2-) which has trapped a positive charge) and Mn(2+) in calcite were singled out for analysis because they are unambiguously interpretable and relatively easy to record. ESR signal strengths of calcite-related SO3(1-) and Mn(2+) have been studied as functions of stratigraphic position in whole-rock samples across the KT boundary at Sopelana, Caravaca, and Blake Nose. At all three of these sites, anomalies in SO3(1-) and/or Mn(2+) intensities are noted at the KT boundary relative to the corresponding background levels in the rocks above and below. At Caravaca, the SO3(1-) background itself is found to be lower by a factor of 2.7 in the first 30,000 years of the Tertiary relative to its steady-state value in the last 15,000 years of the Cretaceous, indicating either an abrupt and quasi-permanent change in ocean chemistry (or temperature) or extinction of the marine biota primarily responsible for fixing sulfite in the late Cretaceous limestones. An exponential decrease in the Mn(2+) concentration per unit mass calcite, [Mn(2+)], as the KT boundary at Caravaca is approached from below (1/e characteristic length =1.4 cm) is interpreted as a result of post-impact leaching of the seafloor. Absolute ESR quantitative analyses of proximal impact deposits from Belize and southern Mexico group naturally into three distinct fields in a twodimensional [SO3(1-)]-versus-[Mn(2+)] scatter plot. These fields contain (I) limestone ejecta clasts, (II) accretionary lapilli, and (III) a variety of SO3(1-) -depleted/Mn(2+) enriched impact deposits. Data for the investigated non-impact-related Cretaceous and Tertiary marine limestones (Spain and Blake Nose) fall outside of these three fields. With reference to thes enon-impact deposits, fields I, II, and III can be respectively characterized as Mn(2+) -depleted, SO3(1-) -enhanced, and SO3(1-) -depleted. It is proposed that (1) field I represents calcites from the Yucatin Platform, and that the Mn(2+) -depleted signature can be used as an indicator of primary Chicxulub ejecta in deep marine environments and (2) field II represents calcites that include a component formed in the vapor plume, either from condensation in the presence of CO2/SO3(1-) -rich vapors, or reactions between CaO and CO2/SO3 rich vapors, and that this SO3(1-) -enhanced signature can be used as an indicator of impact vapor plume deposits. Given these two propositions, the ESR data for the Blake Nose deposits are ascribed to the presence of basal coarse calcitic Chicxulub ejecta clasts, while the finer components that are increasingly represented toward the top are interpreted to contain high- SO3(1-) calcite from the vapor plume. The apparently-undisturbed Bass River deposit may contain even higher concentrations of vapor-plume calcite. None of the three components included in field III appear to be represented at distal, deep marine KT-boundary sites; this field may include several types of impact-related deposits of diverse origins and diagenetic histories.