Kenneth G. MacLeod

Deep-sea paleotemperature record of extreme warmth during the Cretaceous

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.

Timing of mammal-like reptile extinctions across the Permian-Triassic boundary in South Africa

The rate, timing, and pattern of change in different regions and paleoenvironments are critical for distinguishing among potential causes for the Permian-Triassic (P-T) extinction. Carbon isotopic stratigraphy can provide global chronostratigraphic control. We report a large δ 13 C excursion at the P-T boundary and no long-term Permian δ 13 C trends for samples from the interior of Pangea. Stratigraphic gaps between available samples limit the resolution of our δ 13 C curve, but the excursion is within a 15-m-thick zone of overlap between Permian and Triassic taxa. Sedimentological and taphonomic observations demonstrate that this 15 m interval does not represent geologically instantaneous deposition. Together these data support a rapid and globally synchronous P-T event, but suggest that it occurred over a geologically resolvable interval of time.

Nd isotopic excursion across Cretaceous ocean anoxic event 2 (Cenomanian-Turonian) in the tropical North Atlantic

Late Cretaceous fish debris from Demerara Rise exhibits a dramatic positive excursion of 8 ϵ Nd units during ocean anoxic event 2 (OAE2) that is superimposed on extremely low ϵ Nd(t) values (−14 to −16.5) observed throughout the rest of the studied interval. The OAE2 ϵ Nd excursion is the largest yet documented in marine sediments, and the majority of the shift is estimated to have occurred over Nd values on Demerara Rise are explained as the Nd isotopic signature of the South American craton, whereas eruptions of the Caribbean large igneous province or enhanced mixing of intermediate waters in the North Atlantic could have caused the excursion.

North Atlantic warming during global cooling at the end of the Cretaceous

Differences in regional responses to climate fluctuations are well documented on short time scales (e.g., El Nino–Southern Oscillation), but with the exception of latitudinal temperature gradients, regional patterns are seldom considered in discussions of ancient greenhouse climates. Contrary to the expectation of global warming or global cooling implicit in most treatments of climate evolution over millions of years, this paper shows that the North Atlantic warmed by as much as 6 °C (1.5‰ decrease in δ18O values of planktic foraminifera) during the Maastrichtian global cooling interval. We suggest that warming was the result of the importation of heat from the South Atlantic. Decreasing North Atlantic δ18O values are also associated with increasing gradients in planktic δ13C values, suggesting increasing surface-water stratification and a correlated strengthening of the North Atlantic Polar Front. If correct, this conclusion predicts arctic cooling during the late Maastrichtian. Beyond implications for the Maastrichtian, these data demonstrate that climate does not behave as if there is a simple global thermostat, even on geologic time scales.

OXYGEN ISOTOPIC COMPOSITION OF BIOGENIC PHOSPHATE AND THE TEMPERATURE OF EARLY ORDOVICIAN SEAWATER

Abstract Stable isotopic values were measured on micrite, sparry calcite, dolomite, inarticulated brachiopods, and conodonts from the Lange Ranch section (central Texas) of the Lower Ordovician Tanyard Formation. The section spans the upper Cordylodus angulatus Zone through the lower Rossodus manitouensis Zone. An ∼2‰ negative δ13C shift from >0‰ to <−1.5‰VPDB through the section suggests the lower third of the Rossodus manitouensis Zone was sampled. Consistent with previous studies, the δ18O values of carbonates are low, ranging from −3.3‰ to −8.1‰VPDB. Phosphate δ18O values range from 15.4‰ to 17.1‰VSMOW. Paleotemperature estimates calculated from micrite δ18O values assuming an ice-free seawater δ18O value of −1‰VSMOW indicate Early Ordovician tropical seawater temperatures averaged 42°C, whereas δ18O values of co-occurring biogenic phosphate assuming the same seawater value yield paleotemperature estimates averaging 37°C. The phosphate values are interpreted as less affected by diagenesis than carbonate values and suggest Early Ordovician tropical paleotemperatures were not more than 10°C warmer or the oxygen isotopic composition of Early Ordovician hydrosphere was not more than 2‰ lower than present.

Impact and extinction in remarkably complete Cretaceous-Tertiary boundary sections from Demerara Rise, tropical western North Atlantic

Ocean Drilling Program (ODP) Leg 207, on the Demerara Rise in the western tropical North Atlantic, recovered multiple Cretaceous-Paleogene boundary sections containing an ejecta layer. Sedimentological, geochemical, and paleontological changes across the boundary closely match patterns expected for a mass extinction caused by a single impact. A normally graded, ~2-cmthick bed of spherules that is interpreted as a primary air-fall deposit of impact ejecta occurs between sediments of the highest Cretaceous Plummerita hantkeninoides foraminiferal zone and the lowest Paleogene P0 foraminiferal zone. There are no other spherule layers in the section. In addition to extinction of Cretaceous taxa, foraminiferal abundance drops from abundant to rare across the boundary. Ir concentrations reach a maximum of ~1.5 ppb at the top of the spherule bed, and the Ir anomaly is associated with enrichment in other siderophile elements. We attribute the unusually well-preserved and relatively simple stratigraphy to the fact that Demerara Rise was close enough (~4500 km) to the Chicxulub impact site to receive ~2 cm of ejecta, yet was far enough away (and perhaps sheltered by the curve of northern South America) to have been relatively unaffected by impact-induced waves.

Bioturbation, inoceramid extinction, and mid-Maastrichtian ecological change

Notable among the biological changes of the middle part of the Maastrichtian Age was a major pulse of extinction among inoceramid bivalves. Analysis of the distribution of inoceramid shell fragments suggests that the population of burrowing organisms increased across the same stratigraphic interval that the bivalve population decreased. A global reorganization of ocean circulation that resulted in cooler, better oxygenated bottom waters is proposed as an explanation for the observed changes in the deep sea. Other observations regarding the mid-Maastrichtian are consistent with this hypothesis.

Evidence that inoceramid bivalves were benthic and harbored chemosynthetic symbionts

Inoceramid bivalves have been interpreted as both benthic and pseudoplanktonic. The comparison of 18O/16O and 13C/12C signatures of inoceramid shells with surface-dwelling and bottom-dwelling organisms should provide a simple means of resolving the controversy; however, we have found that the stable- isotope pattern is ambiguous. The bivalves have oxygen values similar to their contemporary benthic foraminifera but have carbon values similar to their contemporary planktonic foraminifera. We attempt to resolve this paradox by interpreting those inoceramids analyzed as benthic organisms that harbored chemosynthetic symbionts. A similar pattern of heavy oxygen and heavy carbon values is found in shell carbonate of some modern bivalves with chemosynthetic symbionts living around cold seeps off the coast of Oregon.

A stable and hot Turonian without glacial δ18O excursions is indicated by exquisitely preserved Tanzanian foraminifera

A shift from the icehouse climate in which humans evolved to a Late Cretaceous–like greenhouse climate is an often-repeated cautionary prediction of the consequences of continued anthropogenic CO 2 emissions. The corollary, that understanding the past might help predict the future, has justified many Late Cretaceous studies, but important questions remain about climate stability and sensitivity. New δ 18 O measurements of more than 1000 samples of exceptionally well preserved foraminifera (8 planktic and 11 benthic taxa) from two sites in Tanzania indicate that hot and remarkably stable conditions prevailed in the region during the Turonian, including during a proposed greenhouse glacial event. Planktic taxa have δ 18 O values largely between –4.0‰ and –5.0‰, suggesting surface-water temperatures between 30 and 35 °C. Estimates for seafloor temperatures are between 18 and 25 °C. No parallel shifts in δ 18 O values are observed among planktic and benthic taxa, contradicting an often-cited line of evidence for greenhouse glaciations and supporting an effectively ice-free Turonian world.

Continental warming preceding the Palaeocene-Eocene thermal maximum.

Marine and continental records show an abrupt negative shift in carbon isotope values at ∼55.8 Myr ago. This carbon isotope excursion (CIE) is consistent with the release of a massive amount of isotopically light carbon into the atmosphere and was associated with a dramatic rise in global temperatures termed the Palaeocene–Eocene thermal maximum (PETM). Greenhouse gases released during the CIE, probably including methane, have often been considered the main cause of PETM warming. However, some evidence from the marine record suggests that warming directly preceded the CIE, raising the possibility that the CIE and PETM may have been linked to earlier warming with different origins. Yet pre-CIE warming is still uncertain. Disentangling the sequence of events before and during the CIE and PETM is important for understanding the causes of, and Earth system responses to, abrupt climate change. Here we show that continental warming of about 5 °C preceded the CIE in the Bighorn Basin, Wyoming. Our evidence, based on oxygen isotopes in mammal teeth (which reflect temperature-sensitive fractionation processes) and other proxies, reveals a marked temperature increase directly below the CIE, and again in the CIE. Pre-CIE warming is also supported by a negative amplification of δ13C values in soil carbonates below the CIE. Our results suggest that at least two sources of warming—the earlier of which is unlikely to have been methane—contributed to the PETM.