Benjamin S. Cramer

The Phanerozoic Record of Global Sea-Level Change

We review Phanerozoic sea-level changes [543 million years ago (Ma) to the present] on various time scales and present a new sea-level record for the past 100 million years (My). Long-term sea level peaked at 100 ± 50 meters during the Cretaceous, implying that ocean-crust production rates were much lower than previously inferred. Sea level mirrors oxygen isotope variations, reflecting ice-volume change on the 104- to 106-year scale, but a link between oxygen isotope and sea level on the 107-year scale must be due to temperature changes that we attribute to tectonically controlled carbon dioxide variations. Sea-level change has influenced phytoplankton evolution, ocean chemistry, and the loci of carbonate, organic carbon, and siliciclastic sediment burial. Over the past 100 My, sea-level changes reflect global climate evolution from a time of ephemeral Antarctic ice sheets (100 to 33 Ma), through a time of large ice sheets primarily in Antarctica (33 to 2.5 Ma), to a world with large Antarctic and large, variable Northern Hemisphere ice sheets (2.5 Ma to the present).

Ocean overturning since the Late Cretaceous: Inferences from a new benthic foraminiferal isotope compilation

[1] Benthic foraminiferal oxygen isotopic (d 18 O) and carbon isotopic (d 13 C) trends, constructed from compilations of data series from multiple ocean sites, provide one of the primary means of reconstructing changes in the ocean interior. These records are also widely used as a general climate indicator for comparison with local and more specific marine and terrestrial climate proxy records. We present new benthic foraminiferal d 18 O and d 13 C compilations for individual ocean basins that provide a robust estimate of benthic foraminiferal stable isotopic variations to � 80Ma andtentatively to � 110Ma. First-order variations ininterbasinal isotopicgradients delineate transitions from interior ocean heterogeneity during the Late Cretaceous (>� 65 Ma) to early Paleogene (35– 65 Ma) homogeneity and a return to heterogeneity in the late Paleogene–early Neogene (35–0 Ma). We propose that these transitions reflect alterations in a first-order characteristic of ocean circulation: the ability of winds to make water in the deep ocean circulate. We document the initiation of large interbasinal d 18 O gradients in the early Oligocene and link the variations in interbasinal d 18 O gradients from the middle Eocene to Oligocene with the increasing influence of wind-driven mixing due to the gradual tectonic opening of Southern Ocean passages and initiation and strengthening of the Antarctic Circumpolar Current. The role of wind-driven upwelling, possibly associated with a Tethyan Circumequatorial Current, in controlling Late Cretaceous interior ocean heterogeneity should be the subject of further research.

A case for a comet impact trigger for the Paleocene/Eocene thermal maximum and carbon isotope excursion

Abstract We hypothesize that the rapid onset of the carbon isotope excursion (CIE) at the Paleocene/Eocene boundary (∼55 Ma) may have resulted from the accretion of a significant amount of 12C-enriched carbon from the impact of a ∼10 km comet, an event that would also trigger greenhouse warming leading to the Paleocene/Eocene thermal maximum and, possibly, thermal dissociation of seafloor methane hydrate. Indirect evidence of an impact is the unusual abundance of magnetic nanoparticles in kaolinite-rich shelf sediments that closely coincide with the onset and nadir of the CIE at three drill sites on the Atlantic Coastal Plain. After considering various alternative mechanisms that could have produced the magnetic nanoparticle assemblage and by analogy with the reported detection of iron-rich nanophase material at the Cretaceous/Tertiary boundary, we suggest that the CIE occurrence was derived from an impact plume condensate. The sudden increase in kaolinite is thus thought to represent the redeposition on the marine shelf of a rapidly weathered impact ejecta dust blanket. Published reports of a small but significant iridium anomaly at or close to the Paleocene/Eocene boundary provide supportive evidence for an impact.

Impact of Antarctic Circumpolar Current Development on Late Paleogene Ocean Structure

The modern four-layered ocean structure developed during the early Oligocene, when Antarctica developed permanent ice cover. Global cooling and the development of continental-scale Antarctic glaciation occurred in the late middle Eocene to early Oligocene (~38 to 28 million years ago), accompanied by deep-ocean reorganization attributed to gradual Antarctic Circumpolar Current (ACC) development. Our benthic foraminiferal stable isotope comparisons show that a large δ13C offset developed between mid-depth (~600 meters) and deep (>1000 meters) western North Atlantic waters in the early Oligocene, indicating the development of intermediate-depth δ13C and O2 minima closely linked in the modern ocean to northward incursion of Antarctic Intermediate Water. At the same time, the ocean’s coldest waters became restricted to south of the ACC, probably forming a bottom-ocean layer, as in the modern ocean. We show that the modern four-layer ocean structure (surface, intermediate, deep, and bottom waters) developed during the early Oligocene as a consequence of the ACC.

Uncorking the bottle: What triggered the Paleocene/Eocene thermal maximum methane release?

The Paleocene/Eocene thermal maximum (PETM) was a time of rapid global warming in both marine and continental realms that has been attributed to a massive methane (CH 4 ) release from marine gas hydrate reservoirs. Previously proposed mechanisms for this methane release rely on a change in deepwater source region(s) to increase water temperatures rapidly enough to trigger the massive thermal dissociation of gas hydrate reservoirs beneath the seafloor. To establish constraints on thermal dissociation, we model heat flow through the sediment column and show the effect of the temperature change on the gas hydrate stability zone through time. In addition, we provide seismic evidence tied to borehole data for methane release along portions of the U.S. continental slope; the release sites are proximal to a buried Mesozoic reef front. Our model results, release site locations, published isotopic records, and ocean circulation models neither confirm nor refute thermal dissociation as the trigger for the PETM methane release. In the absence of definitive evidence to confirm thermal dissociation, we investigate an alternative hypothesis in which continental slope failure resulted in a catastrophic methane release. Seismic and isotopic evidence indicates that Antarctic source deepwater circulation and seafloor erosion caused slope retreat along the western margins of the North Atlantic in the late Paleocene. Continued erosion or seismic activity along the oversteepened continental margin may have allowed methane to escape from gas reservoirs trapped between the frozen hydrate-bearing sediments and the underlying buried Mesozoic reef front, precipitating the Paleocene/Eocene boundary methane release. An important implication of this scenario is that the methane release caused (rather than resulted from) the transient temperature increase of the PETM. Neither thermal dissociation nor mechanical disruption of sediments can be identified unequivocally as the triggering mechanism for methane release with existing data. Further documentation with high-resolution benthic foraminiferal isotopic records and with seismic profiles tied to borehole data is needed to clarify whether erosion, thermal dissociation, or a combination of these two was the triggering mechanism for the PETM methane release.

An Exceptional Chronologic, Isotopic, and Clay Mineralogic Record of the Latest Paleocene Thermal Maximum, Bass River, NJ, ODP 174AX

A thick, apparently continuous section recording events of the latest Paleocene thermal maximum in a neritic setting was drilled at Bass River State Forest, New Jersey as part of ODP Leg 174AX [Miller, Sugarman, Browning et al., 1998]. Integrated nannofossil and magneto-stratigraphy provides a firm chronology supplemented by planktonic foraminiferal biostratigraphy. This chronologic study indicates that this neritic section rivals the best deep-sea sections in providing a complete record of late Paleocene climatic events. Carbon and oxygen isotopes measured on benthic foraminifera show a major (4.0% in carbon, 2.3% in oxygen) negative shift correlative with the global latest Paleocene carbon isotope excursion (CIE). A sharp increase in kaolinite content coincides with the isotope shift in the Bass River section, analogous to increases found in several other records. Carbon and oxygen isotopes remain low and kaolinite content remains high for the remainder of the depositional sequence above the CIE (32.5 ft, 9.9 m), which we estimate to represent 300-500 k.y. We interpret these data as indicative of an abrupt shift to a warmer and wetter climate along the North American mid-Atlantic coast, in concert with global events associated with the CIE.

Bipolar Atlantic deepwater circulation in the middle‐late Eocene: Effects of Southern Ocean gateway openings

We present evidence for Antarctic Circumpolar Current (ACC)-like effects on Atlantic deepwater circulation beginning in the late-middle Eocene. Modern ocean circulation is characterized by a thermal differentiation between Southern Ocean and North Atlantic deepwater formation regions. In order to better constrain the timing and nature of the initial thermal differentiation between Northern Component Water (NCW) and Southern Component Water (SCW), we analyze benthic foraminiferal stable isotope (δ18Obf and δ13Cbf) records from Ocean Drilling Program Site 1053 (upper deep water, western North Atlantic). Our data, compared with published records and interpreted in the context of ocean circulation models, indicate that progressive opening of Southern Ocean gateways and initiation of a circum-Antarctic current caused a transition to a modern-like deep ocean circulation characterized by thermal differentiation between SCW and NCW beginning ~38.5 Ma, in the initial stages of Drake Passage opening. In addition, the relatively low δ18Obf values recorded at Site 1053 show that the cooling trend of the middle-late Eocene was not global, because it was not recorded in the North Atlantic. The timing of thermal differentiation shows that NCW contributed to ocean circulation by the late-middle Eocene, ~1–4 Myr earlier than previously thought. We propose that early NCW originated in the Labrador Sea, based on tectonic reconstructions and changes in foraminiferal assemblages in this basin. Finally, we link further development of meridional isotopic gradients in the Atlantic and Pacific in the late Eocene with the Tasman Gateway deepening (~34 Ma) and the consequent development of a circumpolar proto-ACC.

Latest Paleocene–earliest Eocene cyclostratigraphy: using core photographs for reconnaissance geophysical logging

Abstract I present a new method for reconnaissance cyclostratigraphic study of continuously cored boreholes: the generation of detailed sediment color logs by digitizing Ocean Drilling Program (ODP) core photographs. The reliability of the method is tested by comparison with the spectrophotometer color log for the uppermost Paleocene–lowermost Eocene section (Chron C24r) at ODP Hole 1051A. The color log generated from Hole 1051A core photographs is essentially identical to the spectrophotometer log. The method is applied to the Chron C24r section at Holes 1051A and 690B, producing the first high-resolution geophysical log for the latter section. I calculate astronomically calibrated durations between bio- and chemostratigraphic events within Chron C24r by correlating the cyclostratigraphies for Holes 1051A and 690B. These durations are significantly different from previous estimates, indicating that the chronology of events surrounding the Paleocene/Eocene boundary will have to be revised. This study demonstrates that useful geophysical logs can be generated from digitized ODP and DSDP core photographs. The method is of practical use for sections lacking high-resolution logs, as is the case for most lower Paleogene sections.

Reply to a comment on ‘‘A case for a comet impact trigger for the Paleocene/Eocene thermal maximum and carbon isotope excursion’’ by G.R. Dickens and J.M. Francis

Contrary to Dickens and Francis’s claim [1] that we ‘challenge the idea of a massive CH4 release during the PETM (Paleocene/Eocene thermal maximum)’, our consideration of an extraterrestrial carbon contribution to the carbon isotope excursion (CIE) is speci¢cally limited to the initial and most rapid decrease in N13C, which accounts for less than half of the full magnitude of the CIE [2]. Thermal dissociation in response to the warming at the PETM is explicitly allowed in our hypothesis, as reiterated in our conclusions that the impact ‘may have triggered a more gradual thermal dissociation of sea£oor methane hydrates’ [2]. We directly challenge only that portion of the hydrate dissociation hypothesis that relies on gradual warming intrinsic to Earth’s climate system as the triggering mechanism [3]. Such a mechanism is not consistent with the documented essentially synchronous and instantaneous warming and decrease in N13C values at the onset of the event [3,4] and is also at odds with the occurrence of the CIE during an interval of low amplitude orbital forcing of climate [5]. Instead, we postulate a comet impact as an explanation for the rapid onset of the event. Dickens and Francis state that the ‘primary di⁄culty with invoking a comet T is that there is no supporting evidence’ and then list four points from our paper that, taken out of context, are construed as damaging to our hypothesis : (1) ‘There is no crater’. If this were to be taken as a fatal problem with hypothesizing an impact then the idea of a K/T (Cretaceous/Tertiary) impact would never have gained any traction ^ it took 10 years to identify the smoking gun at Chicxulub crater [6^8]. In addition, it is hardly ‘contrived’ to acknowledge that the P/E (Paleocene/ Eocene) impact may have occurred on oceanic crust, which constitutes more than half of Earth’s surface area and where impact craters of any age have been very di⁄cult to ¢nd. (2) ‘The remarkable fossil turnovers T strongly contrast to those across the Cretaceous/Tertiary Boundary T.’ In fact, although we noted that the two events are ‘clearly diierent’ [2] and that an a priori assumption that big impacts should be

Neritic records of the late Paleocene thermal maximum from New Jersey

Boreholes drilled onshore in the New Jersey Coastal Plain recovered a neritic record of the transition to the early Eocene climatic optimum, including complete sections through the late Paleocene thermal maximum (LPTM) drilled at Bass River and Ancora. These New Jersey sections have good to excellent preservation of microfossils, allowing detailed studies of faunal changes among planktonic and benthic foraminifera and calcareous nannoplankton. Biostratigraphy, magnetostratigraphy, clay mineralogic data, and benthic foraminiferal stable isotope data for the Bass River borehole have been reported in Cramer et al. (in press). We present new calcareous nannoplankton biostratigraphic data from the Ancora borehole and benthic foraminiferal faunal data from the Bass River and Ancora boreholes. Isotopic and faunal variations in the neritic sections are similar in many respects to deep sea records, and reflect the imprint of global climate change. Isotopic records show a c. 4‰ negative shift in δC values (Fig. 1; Ancora isotopic records are currently incomplete). The magnitude of the shift is comparable to the record from ODP Site 690B (Kennett & Stott 1991) and the terrestrial record from the Bighorn Basin (Koch et al. 1992) and provides strong evidence that the New Jersey boreholes preserve complete records of the LPTM. At both Bass River and Ancora, a significant turnover among benthic foraminifera occurs coincident with onset of the LPTM (Fig. 2). The benthic foraminifera Gavelinella beccariiformis (White) is abundant below the LPTM in both sections and disappears at the level of the LPTM, firmly linking the turnover in the neritic benthic assemblage with the benthic foraminiferal extinction event recorded in deep-sea sections (see Thomas 1998). The isotope records from the Bass River site differ from deepsea records in the interval above the onset of the LPTM. Deepsea records show an exponential recovery to near pre-excursion values in both δC and δO records (e.g. ODP Site 690, Kennett & Stott 1991), ODP Site 1051 (Katz et al. 1999 and this volume). In contrast, isotopic values remain low in the interval above the CIE (carbon isotope excursion) at Bass River (Fig. 1). We cannot