Andrew S. Gale

Are we now living in the Anthropocene

The term Anthropocene, proposed and increasingly employed to denote the current interval of anthropogenic global environmental change, may be discussed on stratigraphic grounds. A case can be made for its consideration as a formal epoch in that, since the start of the Industrial Revolution, Earth has endured changes sufficient to leave a global stratigraphic signature distinct from that of the Holocene or of previous Pleistocene interglacial phases, encompassing novel biotic, sedimentary, and geochemical change. These changes, although likely only in their initial phases, are sufficiently distinct and robustly established for suggestions of a Holocene–Anthropocene boundary in the recent historical past to be geologically reasonable. The boundary may be defined either via Global Stratigraphic Section and Point (“golden spike”) locations or by adopting a numerical date. Formal adoption of this term in the near future will largely depend on its utility, particularly to earth scientists working on late Holocene successions. This datum, from the perspective of the far future, will most probably approximate a distinctive stratigraphic boundary.

Chemostratigraphy versus biostratigraphy: data from around the Cenomanian–Turonian boundary

A detailed isotopic profile is presented for a stratigraphically expanded Cenomanian–Turonian boundary section in Chalk facies exposed at Eastbourne, Sussex and compared with data from Pueblo, Colorado. In both sections macro- and micropalaeontological markers (first appearances and disappearances) are well-constrained, and their relative positions and relationship to the structure of the carbon-isotope curve are identical. This consistent relationship between two independent phenomena, one geochemical, the other biostratigraphical, is taken as evidence for the likely synchroneity of both the biostratigraphical markers and the carbon-isotope excursion in these two areas. This interpretation contrasts with suggestions made recently by other authors whose data have been taken to imply a lack of correlation between the carbon-isotope excursion in Europe and North America.

Sea-level change and rock-record bias in the Cretaceous: a problem for extinction and biodiversity studies

Abstract The association between mass extinction in the marine realm and eustatic sea-level change in the Mesozoic is well documented, but perplexing, because it seems implausible that sea-level change could actually cause a major extinction. However, large-scale cycles of sea-level change can and do alter the ratio of shallow to deep marine continental-shelf deposits preserved in the rock record both regionally and globally. This taphonomic megabias alone could be driving patterns of first and last occurrence and standing diversity because diversity and preservation potential both change predictably with water depth. We show that the Cenomanian/Turonian faunal event in western Europe has all the predicted signatures expected if taphonomic megabias was the cause. Grade taxa terminating in pseudoextinction and Lazarus taxa are predominantly found in the onshore facies that disappear for extended periods from the rock record. Before other mass extinctions are taken at face value, a much more careful analysis of biases in the rock record needs to be carried out, and faunal disappearances need to be analyzed within a phylogenetic framework.

Global correlation of Cenomanian (Upper Cretaceous) sequences: Evidence for Milankovitch control on sea level

We have investigated the sequence stratigraphy of two widely separated marine Cenomanian successions in southeast India and northwest Europe, and used high-resolution ammonite biostratigraphy to demonstrate that sea-level changes are globally synchronous and therefore must be eustatically controlled. Sequence-scale sea-level changes in the Cenomanian were driven by the long eccentricity cycle (400 k.y.) in the Milankovitch band. We hypothesize that, during pre-Quaternary time, the third-order sequences of Vail and Haq are essentially a sediment response to sea-level changes driven by the 400 k.y. cycle. Construction of a relative sea-level curve for the marginal marine succession in India demonstrates that the short-term sea-level changes are rapid (10-100 m/m.y.) and have a magnitude of 2-20 m. Glacioeustasy is a possible but unproven driving mechanism.

Marine biodiversity through the Late Cenomanian–Early Turonian: palaeoceanographic controls and sequence stratigraphic biases

Changes in the marine macro- and microfauna, sedimentary geochemistry and surface-water palaeoproductivity through the last 500 000 years of the Cenomanian and first 300 000 years of the Turonian are documented. These are based on the succession at Eastbourne, the thickest and most complete section through the Late Cenomanian and Early Turonian in the Anglo–Paris Basin. Two levels of rapid faunal and geochemical change are identified, one coincident with a significant increase in siliciclastic input at the base of the Plenus Marls Member, and the other with a marked drop in surface water productivity near the top of the same unit. Faunal change is demonstrated to be largely a pattern of immigration–emigration rather than true extinction, and our sequence stratigraphical analysis shows that it was coincident with major sea-level changes. No evidence is found to support the hypothesis that reduced bottom water oxygenation developed and was responsible for extinctions amongst the benthos in mid-shelf environments. The onset of pure chalk facies is interpreted to mark the breakdown of shelf-break fronts and the spread of oligotrophic oceanic waters over much of the continental shelf, initiated by rising sea-level. The Cenomanian–Turonian event, far from recording a mass extinction of shelf fauna, is most probably an artifact caused by a significant switch in the nature of the surviving sedimentary record as a result of a major, but perfectly ordinary, oceanographic change.

Stratigraphy of the Anthropocene

The Anthropocene, an informal term used to signal the impact of collective human activity on biological, physical and chemical processes on the Earth system, is assessed using stratigraphic criteria. It is complex in time, space and process, and may be considered in terms of the scale, relative timing, duration and novelty of its various phenomena. The lithostratigraphic signal includes both direct components, such as urban constructions and man-made deposits, and indirect ones, such as sediment flux changes. Already widespread, these are producing a significant ‘event layer’, locally with considerable long-term preservation potential. Chemostratigraphic signals include new organic compounds, but are likely to be dominated by the effects of CO2 release, particularly via acidification in the marine realm, and man-made radionuclides. The sequence stratigraphic signal is negligible to date, but may become geologically significant over centennial/millennial time scales. The rapidly growing biostratigraphic signal includes geologically novel aspects (the scale of globally transferred species) and geologically will have permanent effects.

Geochemistry of pelagic and hemipelagic carbonates: criteria for identifying systems tracts and sea-level change

The elemental (Si, Ti, Al, Mn, Ca, Zr) and carbon stable-isotope (δ13C) geochemistry of a biostratigraphically well-constrained Cenomanian–Turonian (Upper Cretaceous) Chalk succession on the Isle of Wight, southern England, shows systematic variation that corresponds closely to a published sequence stratigraphic model for the Cenomanian. Six sequences and their constituent systems tracts, defined elsewhere using sedimentological criteria, are clearly distinguishable from bulk-sediment elemental profiles, and an additional Upper Cenomanian sequence previously identified in Spain is recognized in England from these geochemical data. The manganese curve is particularly instructive, exhibiting minima around sequence boundaries and through lowstands, rising values from the transgressive surfaces through transgressive systems tracts, maxima around maximum flooding surfaces, and declining values through highstands. Silica and trace-element (Ti, Zr) aluminium ratios peak around transgressive surfaces and maximum flooding surfaces, indicating pulses of increased siliciclastic input. Positive δ13C excursions are confirmed at the base of the Middle Cenomanian and spanning the Cenomanian–Turonian boundary but are not evident in other sequences. Variation in Mn is related to bulk sedimentation rate and detrital versus biogenic supply, which control the Mn flux and the efficiency of the diagenetic Mn ‘pump’ that leads to elevated Mn contents in sediments. Manganese peaks do not generally correlate with positive δ13C excursions, and although near-coincident Mn and δ13C peaks occur around the Cenomanian–Turonian boundary, the former is not necessarily linked to the oceanic anoxic event occurring at that time. The global oceanic Mn flux may have been enhanced during the Cenomanian as a result of hydrothermal activity during rapid sea-floor spreading and oceanic plateau formation. Elemental chemostratigraphy provides a new tool for developing sequence stratigraphic models in pelagic and hemipelagic carbonate successions.

Eustatic sea-level record for the Cenomanian (Late Cretaceous)—Extension to the Western Interior Basin, USA

A combination of biostratigraphic markers (ammonites, inoceramid bivalves) and carbon isotope excursions is employed to establish a high-resolution correlation between the middle to late Cenomanian successions of the Western Interior Basin (USA) and the Anglo-Paris Basin (southern UK). Sequences identified from sedimentologic criteria in the Pueblo succession and elsewhere in the Western Interior Basin are shown to coincide precisely with globally recognized sea-level events and were therefore under eustatic control. This evidence refutes arguments that Cenomanian sequences in the Western Interior Basin were formed by local tectonic events. The interaction of longer-term tectonic movements and more rapid eustatic change may have simply enhanced the amount of erosion associated with sequence boundaries. A crossplot of radiometric ages derived from North American bentonites against an orbitally tuned time scale developed in the Anglo-Paris Basin provides support for the argument that the sequences were controlled by the 405-k.y.-long eccentricity cycle.

Global correlation of Upper Campanian - Maastrichtian successions using carbon-isotope stratigraphy: development of a new Maastrichtian timescale

Carbon-isotope stratigraphy has proven to be a powerful tool in the global correlation of Cretaceous successions. Here we present new, high-resolution carbon-isotope records for the Global Boundary Stratotype Section and Point (GSSP) of the Maastrichtian stage at Tercis les Bains (France), the Bottaccione and Contessa sections at Gubbio (Italy), and the coastal sections at Norfolk (UK) to provide a global δ13C correlation between shelf-sea and oceanic sites. The new δ13C records are correlated with δ13C-stratigraphies of the boreal chalk sea (Trunch borehole, Norfolk, UK, Lagerdorf-Kronsmoor-Hemmoor section, northern Germany, Stevns-1 core, Denmark), the tropical Pacific (ODP Hole 1210B, Shatsky Rise) and the South Atlantic and Southern Ocean (DSDP Hole 525A, ODP Hole 690C) by using an assembled Gubbio δ13C record as a reference curve. The global correlation allows the identification of significant high-frequency δ13C variations that occur superimposed on prominent Campanian-Maastrichtian events, namely the Late Campanian Event (LCE), the Campanian-Maastrichtian Boundary Event (CMBE), the mid-Maastrichtian Event (MME), and the Cretaceous-Paleogene transition (KPgE). The carbon-isotope events are correlated with the geomagnetic polarity scale recalculated using the astronomical 40Ar/39Ar calibration of the Fish Canyon sanidine. This technique allows the evaluation of the relative timing of base occurrences of stratigraphic index fossils such as ammonites, planktonic foraminifera and calcareous nannofossils. Furthermore, the Campanian-Maastrichtian boundary, as defined in the stratotype at Tercis, can be precisely positioned relative to carbon-isotope stratigraphy and the geomagnetic polarity timescale. The average value for the age of the Campanian-Maastrichtian boundary is 72.1 ± 0.1 Ma, estimated by three independent approaches that utilize the Fish Canyon sanidine calibration and Option 2 of the Maastrichtian astronomical timescale. The CMBE covers a time span of 2.5 Myr and reflects changes in the global carbon cycle probably related to tectonic process rather than to glacio-eustasy. The duration of the high-frequency δ13C variations instead coincides with the frequency band of long eccentricity, indicative of orbital forcing of changes in climate and the global carbon cycle.

Petrology and palaeoenvironmental significance of glaucony in the Eocene succession at Whitecliff Bay, Hampshire Basin, UK

Following the investigations of Odin and others into the distribution of green granules, glaucony has been widely assumed to be a reliable indicator of a fully marine, open shelf environment with a low sedimentation rate. We have investigated the value of glaucony as a palaeoenvironmental indicator through an investigation of the pellets, and their distribution and reworking in the predominantly brackish to shallow marine Tertiary sediments of the Hampshire basin, together with a re-evaluation of the sedimentology. Glaucony has apparently formed in situ in all lithofacies from shallow marine to estuarine. Of the three highest glaucony concentrations (all dominated by in situ glaucony) two occur within highstand system tracts, the third is at a sequence boundary. Several important surfaces do not have more than a few percent glaucony, with very variable proportions of mature and in situ pellets. The correlation between glaucony concentration and sequence stratigraphy is most obvious in the London Clay and Wittering Formations, where least reworking of pellets has occurred. In the Barton Group there are no major concentrations of glaucony at any of the important stratal surfaces, we believe this more random glaucony distribution is due to limited glaucony formation and reworking of older glaucony. In these sediments ideal conditions for glaucony formation are interpreted to have been: fully marine, 10–30 m water depth, a ‘warm’ temperature plus low sedimentation rate with periodic winnowing to concentrate the pellets. Although most of these conditions for glaucony formation occurred in the Selsey Formation and Barton Group, a factor or factors mitigated against glauconitization. We suggest that this was lowering of the water temperature. The London Clay and Wittering Formations were deposited relatively rapidly (50–60 m Ma −1) and include intervals of estuarine sedimentation, both factors that we believe inhibited glaucony formation. Glaucony maturity reflects the minimum length of time spent in surface sediments, close to the oxic/sub-oxic interface. Point count data and chemical data for glaucony indicate widespread reworking and an overall increase in reworking with time, possibly due to uplift on the Isle of Wight monocline. The apparently wide range of conditions in which glaucony will form, and the frequency with which it is reworked, suggest that it is a less useful indicator palaeo-environmental indicator than is commonly supposed.