Mike A. Hall

Warm tropical sea surface temperatures in the Late Cretaceous and Eocene epochs

Climate models with increased levels of carbon dioxide predict that global warming causes heating in the tropics, but investigations of ancient climates based on palaeodata have generally indicated cool tropical temperatures during supposed greenhouse episodes. For example, in the Late Cretaceous and Eocene epochs there is abundant geological evidence for warm, mostly ice-free poles, but tropical sea surface temperatures are generally estimated to be only 15–23u2009°C, based on oxygen isotope palaeothermometry of surface-dwelling planktonic foraminifer shells. Here we question the validity of most such data on the grounds of poor preservation and diagenetic alteration. We present new data from exceptionally well preserved foraminifer shells extracted from impermeable clay-rich sediments, which indicate that for the intervals studied, tropical sea surface temperatures were at least 28–32u2009°C. These warm temperatures are more in line with our understanding of the geographical distributions of temperature-sensitive fossil organisms and the results of climate models with increased CO2 levels.

Carbon isotope records from pacific surface waters and atmospheric carbon dioxide

We have stacked planktonic carbon isotope data from three cores in the western equatorial Pacific in order to generate a new reconstruction of atmospheric carbon dioxide over the past 450,000 years. Our new reconstruction resembles that of Shackleton et al. (1983) based on data from East Pacific core V19-30, which successfully predicted features that were subsequently verified by Barnola et al. (1987) in the record from the Vostock ice core. In addition the new data confirm the discovery of Shackleton and Pisias (1985) that changes in atmospheric CO2 lead changes in ice volume and hence probably contributed to the glacial-interglacial cycles. Our new reconstruction avoids some of the deficiencies of the previous reconstruction: in particular the planktonic species (Neogloboquadrina dutertrei), on which the earlier reconstruction depends, does not calcify in truly nutrient-free surface water as the model assumes, whereas our new reconstruction uses Globigerinoides sacculifer which is expected to be more reliable. In addition, the surface waters in the west Pacific are closer to the nutrient-free ideal on which the model (Broecker, 1982) depends. On the other hand, the amplitude of the new reconstruction is significantly smaller than the amplitude observed by Barnola et al. (1987). It is not clear whether this smaller range is a better estimate of the amplitude of the ‘biological pump’ effect, or whether the true amplitude is reduced by bioturbation in the west Pacific cores that we studied.

Stable isotope evidence for foraminiferal habitats during the during of the Cenomanian/Turonian ocean anoxic event

Oxygen and carbon isotope analyses of several species of size-controlled planktonic and benthic foraminifera from the Plenus Marls of Dover (Kent, England) show a clear separation in oxygen and carbon isotope ratios consistent with different habitats; the analyses indicate the existence of vertical gradients of temperature and 13 C in dissolved CO 2 in the Chalk Sea. Whereas sequential ontogenetic stages of planktonic foraminifera do not show systematic variations in δ 18 O or δ 13 C values (implying no systematic change in depth habitat during the growth of individual species), species of the genera Hedbergella and Dicarinella are interpreted to have lived nearer to the surface than Rotalipora . Because vertical 13 C gradients in the Chalk Sea remained stable during the late Cenomanian, the oxygen-minimum zone is unlikely to have impinged on this area.

Hantkeninid depth adaptation: An evolving life strategy in a changing ocean

The interplay between evolution, paleoecology, and environmental change is examined in a geochemical study of a group of Eocene planktonic foraminifera. The hantkeninids, which are well-known biostratigraphic inde × fossils, underwent spectacular long-term evolution in the middle and upper Eocene (49.0–33.7 Ma), a time when major global climate and oceanic changes were occurring. We use oxygen and carbon isotope analysis of their shell calcite to investigate how their habitat changed as they evolved. The hantkeninids originated in a deep-water oxygen-minimum environment, but migrated into fully oxygenated near-surface waters as global temperatures decreased and water-column stratification declined. This change in depth ecology coincided with pronounced morphological evolution, involving changes in chamber shape and degree of inflation, and modification of the primary aperture. These developments are considered to be adaptations to a near-surface habitat.

Stable isotopic evidence for the sympatric divergence of Globigerinoides trilobus and Orbulina universa (planktonic foraminifera)

Two extant species of spinose planktonic foraminifera, Globigerinoides trilobus and Orbulina universa, shared a common ancestor in the early Miocene, as studied here in well-preserved fossil material from Limalok Guyot in the tropical Pacific (Ocean Drilling Program Site 871). The first appearance of O. universa (15.1 Ma) was preceded by an increase in the morphological variance of the ancestral lineage, including the origin of several new and short-ranging Praeorbulina morphospecies. Biological speciation (cladogenesis) probably occurred before 15.1 Ma. New oxygen (δ18O) and carbon (δ 13C) stable isotopic results are compared with analyses of two reference species, Globigerinoides ruber (shallow water) and Globoquadrina venezuelana (deep water). Oxygen isotopic ratios of G. trilobus, Praeorbulina spp., and O. universa indicate that the entire evolutionary transition took place within mixed layer habitats similar to those occupied by modern G. trilobus. A slight separation in the δ 18O of O. universa and G. trilobus later in the mid-Miocene may indicate subsequent habitat partitioning. Carbon isotopes suggest that no significant change in the carbon metabolism or degree of photosymbiosis occurred. The origin of Orbulina therefore appears to have been a case of sympatric speciation at shallow depths in the open ocean. The causes of the speciation and morphological transition are unknown.

Preparation of micro- and crypto-tephras for quantitative microbeam analysis

Abstract The smallest tephra grains present special challenges in their preparation for quantitative microbeam chemical analysis by EPMA and LA-ICP-MS. High-quality polished surfaces are an essential pre-requisite for the collection of high quality quantitative chemical data, as these shards also present significant difficulties for analysis. A method for the preparation of tephra grains as small as 10–50 µm is described in detail. This method uses inexpensive, widely available materials and is easily implemented in facilities in which polished geological samples are prepared for microanalysis. Samples prepared in this way reduce or eliminate the difficulties in microbeam analysis associated with small grains, when appropriate analytical protocols are used.

erratum: Warm tropical sea surface temperatures in the Late Cretaceous and Eocene epochs

This corrects the article DOI: 35097000

Palaeoenvironmental records from the West Antarctic Peninsula drift sediments over the last 75 ka

Abstract We present results of a multi-proxy study on marine sediment core JR179-PC466 recovered from the crest of a sediment drift off the West Antarctic Peninsula at approximately 2300 m water depth. The 10.45 m-long core consists dominantly of glaciomarine terrigenous sediments, with only traces of calcium carbonate (<1 wt%). Despite the very low abundance of calcareous foraminifera, planktonic shell numbers are sufficient for stable isotope analyses in two-thirds of the samples studied. The core chronology is based on oxygen isotope stratigraphy and correlation of its relative palaeomagnetic intensity (RPI) with a stacked reference curve. According to the age model, core PC466 spans the last 75 ka, with average sedimentation rates of between about 4 and 25 cm ka−1. Planktonic foraminifera abundances fluctuate between 0 and 30 individuals per gram throughout the core, with minima observed during Marine Isotope Stage (MIS) 2 (14–29 ka before present, BP) and MIS4 (57–71 ka BP). Planktonic foraminifera are present in the Holocene but more abundant in sediments deposited during MIS3 (29–57 ka BP), owing to less dilution by terrigenous detritus and/or better carbonate preservation. During MIS3, foraminifera maxima correlate with Antarctic warming events as recorded in the δ18O signal of the EPICA Dronning Maud Land (EDML) ice core. They indicate higher planktonic foraminifera production and better carbonate preservation west of the Antarctic Peninsula during that time. The abundance of ice-rafted detritus (IRD) in core PC466 increased during the last deglaciation between about 19 and 11 ka BP, when numerous icebergs drifted across the core site, thereby releasing IRD. During this time, sea-level rise destabilized the Antarctic Peninsula (APIS) and West Antarctic (WAIS) ice sheets that had advanced onto the shelf during the sea-level low-stand of the Last Glacial Maximum (LGM; c. 19–23 ka BP). Overall, our results demonstrate that it is possible to establish an age model and reconstruct palaeoceanographical and climatic changes at high temporal resolution from sedimentary sequences recovered at 2300 m water depth from a West Antarctic drift.