Jerry J. Hooker

Eocene–Oligocene mammalian faunal turnover in the Hampshire Basin, UK: calibration to the global time scale and the major cooling event

A new high-resolution, c. 1 Ma mammalian record in continental Eocene–Oligocene strata in the Hampshire Basin is used to investigate the nature and timing of the major Europe-wide mammalian faunal turnover termed the Grande Coupure. Whether this was caused by climate change or by competition with taxa dispersing from Asia is controversial. The mammalian faunas in this record, after rarefaction analysis, show a sharp reduction in diversity only after the Grande Coupure. Improved correlation of NW European successions to global events confirms the Grande Coupure as earliest Oligocene. It shows that a c. 350 ka hiatus interrupts the Hampshire and Paris Basin sequences prior to the first record of post-Grande Coupure Asian taxa. Hiatus-bridging faunas from elsewhere in Europe record mainly post-Grande Coupure taxa, suggesting that the turnover occurred early in the hiatus, minimizing bias to the turnover pattern. A previously unrecorded, second, smaller turnover, involving European mammals only, immediately precedes the Grande Coupure in the Hampshire Basin, coinciding with vegetational change. This turnover is judged not to represent cooling. It is concluded that the Grande Coupure coincides with the earliest Oligocene Oi-1 glaciation and that climate change combined with competition to produce the turnover.

Summer temperatures of late Eocene to early Oligocene freshwaters

The marine foraminiferal isotope record displays a positive δ18O shift early in the Oligocene, which has been identified as the onset of the Antarctic Oi-1 glaciation. Reported here are the first oxygen isotope–derived summer paleotemperatures for continental freshwater in the Northern Hemisphere (Hampshire Basin, Isle of Wight, UK) leading up to and across this event. These paleotemperatures are derived from multiple paleoproxies (rodent tooth enamel, gastropod shells, charophyte gyrogonites, and fish otoliths) and are independent of freshwater evaporation effects and changes in ice volume. We conclude that a fluctuating mesothermal climate existed, but that there was no significant decrease in summer temperatures across the Oi-1 glaciation. This result is concordant with several other studies in suggesting that the majority of the isotopic shift in the marine realm across the Oi-1 glaciation is linked to changes in Antarctic ice volume and not to global temperature change. Our new approach has allowed us to derive numerical values for summer temperatures as well as to reconstruct relative temperature change across this key interval of the Eocene-Oligocene transition.

Episodic fire, runoff and deposition at the Palaeocene–Eocene boundary

Qualitative and quantitative coal petrological analyses have been undertaken on the laminated lignite at the base of the Cobham Lignite Bed, from Scalers Hill, Kent, England. The maximum negative carbon isotope excursion, which marks the beginning of the Palaeocene–Eocene thermal maximum (PETM), occurs near the top of the laminated lignite. The lignite contains inertinite, a petrographic term used to describe charcoal. The laminated lignite has inertinite-rich and inertinite-poor layers indicative of episodic fires and post-fire erosion. Charcoal clasts are derived from living or recently senesced plants and are dominated by the leaf stalks of herbaceous ferns and wood fragments from flowering plants. The charcoal assemblage reflects a low-diversity flora, possibly adapted to disturbance by fire, derived from a source vegetation subjected to seasonal surface wildfires. The environmental conditions leading up to and across the onset of the PETM are, therefore, interpreted as incorporating a persistent fire regime with episodic wildfires followed by rainfall and runoff events. Abundant charcoal indicates near-modern oxygen levels whereas the absence of charred peat in this area calls into question previous suggestions that burning of Palaeocene peats might have contributed to the short-lived negative carbon isotope excursion at the Palaeocene–Eocene boundary.

Terrestrial cooling in Northern Europe during the eocene-oligocene transition.

Geochemical and modeling studies suggest that the transition from the “greenhouse” state of the Late Eocene to the “icehouse” conditions of the Oligocene 34–33.5 Ma was triggered by a reduction of atmospheric pCO2 that enabled the rapid buildup of a permanent ice sheet on the Antarctic continent. Marine records show that the drop in pCO2 during this interval was accompanied by a significant decline in high-latitude sea surface and deep ocean temperature and enhanced seasonality in middle and high latitudes. However, terrestrial records of this climate transition show heterogeneous responses to changing pCO2 and ocean temperatures, with some records showing a significant time lag in the temperature response to declining pCO2. We measured the Δ47 of aragonite shells of the freshwater gastropod Viviparus lentus from the Solent Group, Hampshire Basin, United Kingdom, to reconstruct terrestrial temperature and hydrologic change in the North Atlantic region during the Eocene–Oligocene transition. Our data show a decrease in growing-season surface water temperatures (∼10 °C) during the Eocene–Oligocene transition, corresponding to an average decrease in mean annual air temperature of ∼4–6 °C from the Late Eocene to Early Oligocene. The magnitude of cooling is similar to observed decreases in North Atlantic sea surface temperature over this interval and occurs during major glacial expansion. This suggests a close linkage between atmospheric carbon dioxide concentrations, Northern Hemisphere temperature, and expansion of the Antarctic ice sheets.

Phosphate δ18O determination of modern rodent teeth by direct laser fluorination: an appraisal of methodology and potential application to palaeoclimate reconstruction

Abstract A direct laser-fluorination (DLF) method is presented for phosphate δ 18 O analysis (mass 1 — 2 mg). The automated system heats samples in the presence of excess BrF 5 using a 25 W CO 2 laser, at 10.66 μm. δ 18 O ratios of the liberated O 2 were measured using a dual inlet Optima mass spectrometer. As DLF measures whole apatite oxygen, non-phosphate bound oxygen must be removed by pre-treatment. Two methods were investigated: 1) heating to 1000°C; and 2) heating to 400°C followed by laser fusing. Method 2 is recommended as samples heated to 1000°C showed evidence of oxygen exchange with atmospheric water. To validate the DLF method, and show the potential of rodent teeth in palaeoclimate reconstruction, modern rodent teeth δ 18 O results from 2 species are presented (δ 18 O p ). Large inter- and intra-jaw heterogeneity indicates that single teeth cannot be used for palaeothermometry. However, the overall standard deviations were low ( Glis glis δ 18 O p = +10.4 ± 0.7‰ n = 38 and Apodemus sylvaticus δ 18 O p = +14.4 ± 1.3‰ n = 24). Using equations, derived from lab rodents, an ingested water value of −5.6 ± 2.2‰ was calculated for Apodemus sylvaticus , only −1.3‰ lower than measured local water (−4.3‰). This suggests that the phosphate δ 18 O of rodent teeth can be used as a proxy for palaeoclimate reconstruction.

Paleogene paleoclimate reconstruction using oxygen isotopes from land and freshwater organisms: the use of multiple paleoproxies

Abstract Understanding past climate change is critical to the interpretation of earth history. Even though relative temperature change has been readily assessed in the marine record, it has been more difficult in the terrestrial record due to restricted taxonomic distribution and isotopic fractionation. This problem could be overcome by the use of multiple paleoproxies. Therefore, the δ18O isotopic composition of five paleoproxies (rodent tooth enamel, δ18OPhosphate = +17.7 ± 2.0‰ n = 74 (VSMOW); fish scale ganoine δ18OPhosphate = +19.7 ± 0.7‰ n = 20 (VSMOW); gastropod shell δ18OCalcite = −1.7 ± 1.3‰ n = 50 (VPDB); charophyte gyrogonite δ18OCalcite = −2.4 ± 0.5‰ n = 20 (VPDB); fish otolith δ18OAragonite = δ18O = −3.6 ± 0.6‰ n = 20 (VPDB)) from the Late Eocene (Priabonian) Osborne Member (Headon Hill Formation, Solent Group, Hampshire Basin, UK) were determined. Because diagenetic alteration was shown to be minimal the phosphate oxygen component of rodent tooth enamel (as opposed to enamel carbonate oxygen) was used to calculate an initial δ18OLocal water value of 0.0 ± 3.4‰. However, a skewed distribution, most likely as a result of the ingestion of evaporating water, necessitated the calculation of a corrected δ18OLocal water value of −1.3 ± 1.7‰ (n = 62). This δ18OLocal water value corresponds to an approximate mean annual temperature of 18 ± 1°C. Four other mean paleotemperatures can also be calculated by combining the δ18OLocal water value with four independent freshwater paleoproxies. The calculated paleotemperature using the fish scale thermometry equations most likely represents the mean temperature (21 ± 2°C) of the entire length of the growing season. This should be concordant with the paleotemperature calculated using the Lymnaea shell thermometry equation (23 ± 2°C). The lack of concordance is interpreted to be the result of diagenetic alteration of the originally aragonitic Lymnaea shell to calcite. The mean paleotemperature calculated using the charophyte gyrogonite thermometry equation (21 ± 2°C), on the other hand, most likely represents the mean temperature of a single month toward the end of the growing season. The fish otolith mean paleotemperature (28 ± 2°C) most likely represents the mean temperature of the warmest months of the growing season. An approximate mean annual temperature of 18 ± 1°C, in addition to a mean growing season paleotemperature of 21 ± 2°C (using fish scale only) with a warmest month temperature of 28 ± 2°C, and high associated standard deviations suggest that a subtropical to warm temperate seasonal climate existed during the deposition of the Late Eocene Osborne Member.

Palynological evidence of vegetation dynamics in response to palaeoenvironmental change across the onset of the Paleocene‐Eocene Thermal Maximum at Cobham, Southern England

A high‐resolution palynological study is undertaken through the Cobham Lignite Bed (Cobham, Kent, UK) to investigate vegetation response to the rapid climate warming at the onset of the Paleocene‐Eocene thermal maximum (PETM). The lower laminated lignite records negative carbon isotope (δ 13C) excursions (CIE) (marking the PETM onset) in bulk organic material, n‐alkanes and, uniquely, also in hopanes. The upper blocky lignite represents an estimated 4–12 kya after PETM onset. Raw and rarefied palynomorph species richness measures are higher in the PETM but the difference is not statistically significant. Only five (of 24) common taxa have last appearance or major shifts in percentage occurrence close to the PETM onset. One of these, a triporate eudicot, occurs only in the maximum negative CIE sample and the immediately underlying sample, the former at very high percentages, an interesting feature of PETM onset. The palynomorph composition of Late Paleocene samples is significantly different from PETM samples. In the late Paleocene there is a close association of high abundances of Cicatricosisporites (Schizaeaceae) fern spores with microscopic and mesoscopic charcoal representing a low diversity fire prone fern and woody angiosperm community. By contrast, the PETM vegetation is characterised by the loss of ferns and cessation of fires, an increase in wetland plants (including cupressaceous conifers) and a more varied flowering plant community with palms and eudicots. These palynofloras thus indicate little response in plant species across the PETM onset but a major change in vegetation composition, linked to a switch in fire regime.

Gnaw marks on Eocene seeds: evidence for early rodent behaviour

Abstract Gnawed holes in fossil seeds of the freshwater aquatic floating plant Stratiotes, collected from the Solent Group of Late Eocene (Priabonian) age at two localities in the Hampshire Basin, UK, are recognised as the oldest direct evidence of seed predation by rodents. The type of gnawing is similar to that made by Wood Mice, Apodemus sylvaticus, today and a more reliable means of differentiating the resultant pattern from those of other gnawing types is described. By using the size relationship between modern gnaw marks and the rodent incisors that made them, isolated fossil incisors from the same strata at sites near to those where the seeds were found are matched closely to the fossil gnaw marks. By comparison with incisors in situ in jaws from the Continental European Palaeogene, and by eliminating other contenders amongst the known Solent Group rodent fauna, it is concluded that the fossil Stratiotes seeds were predated by the extinct glirids Glamys priscus and Glamys devoogdi. Different patterns of gnaw marks associated with different hole sizes are interpreted as different stages in the opening of the seeds and as demonstrating learning behaviour on the part of the predators. Similarity of fossil astragali (ankle bones) from the Solent Group, identified as Glamys devoogdi, to the recent glirid Eliomys quercinus suggests a scansorial rather than arboreal locomotor mode for the former, in keeping with a ground-level foraging strategy as indicated by the habitat of the Stratiotes plant. The direct evidence of feeding from the fossil seeds and the clear link to the specific predators are an important indication of ancient dietary adaptation independent of that deduced from tooth morphology. It supports the change in dominant frugivorous feeding type amongst European rodents from soft fruit-eating pseudoparamyine paramyids and pseudosciurids in the Middle Eocene to hard seed-eating glirids in the Late Eocene, coincident with the global climatic deterioration shown by oxygen isotopes. One new ichnogenus with one new ichnospecies is erected for one type of gnawing in fossil seeds: Glirotremmorpha entectus igen. et isp. nov., from the English Late Eocene.

Discussion on the Eocene–Oligocene boundary in the UK Journal , Vol. 163, 2006, pp. 401–415

Jerry Hooker, Margaret Collinson, Stephen Grimes, Nick Sille & David Mattey write: Recognition of the Eocene–Oligocene boundary in the Hampshire Basin, UK, has been debated since naming of the Oligocene Epoch in 1854. Previously, this was because the boundary itself had not been stabilized and because the strata concerned are largely non-marine. A Global Boundary Stratotype and Stratigraphic Point (GSSP) was established at Massignano, Italy, in 1993 in marine strata. Recognition of the boundary on extinction of the planktonic foraminiferan family Hantkeninidae made boundary identification difficult in the continental realm. Correlation to marginal marine and non-marine strata is nevertheless possible via magnetostratigraphic and sequence stratigraphic studies and, importantly, biostratigraphically via dinocyst zones at Massignano (Brinkhuis & Biffi 1993; Brinkhuis & Visscher 1995). Therefore, recent publication of the magnetostratigraphy, sequence stratigraphy and orbital cyclicity of much of the Hampshire Basin Solent Group (Gale et al . 2006) is welcomed and substantially increases the number of correlation tools available in this area. Such cyclical phenomena, however, rely on absolute dating or biostratigraphy for calibration. No radiometric dates exist for the Solent Group, so biostratigraphy remains the best means of dating the succession. There are, however, problems with the way Gale et al . (2006) have interpreted biostratigraphic markers and therefore with their placement of the Eocene–Oligocene boundary and associated events. The organisms concerned are calcareous nannoplankton (NP zones) and mammals (MP reference levels). Thus, the record by Aubry (1985) of NP22 in the Argiles Vertes de Romainville, Paris Basin, was subsequently qualified by her (Aubry 1986, p. 307) as ‘zone NP22 (not younger; possibly older: NP21?)’. This dating was based solely on the presence of rare Isthmolithus recurvus , which ranges from NP19/20 to NP22 (Aubry 1992), this being the real level of dating for the Argiles Vertes de Romainville …

A new clade of omomyid primates from the European Paleogene

Abstract A new genus, Vectipithex, is erected in the subfamily Microchoerinae, family Omomyidae, for four species, of which one, the type species is new: V. smithorum sp. nov. ‘Protoadapis’ ulmensis is referred to Vectipithex and transferred from the family Adapidae. Two species of Nannopithex, ‘N.’ quaylei and ‘N.’ raabi are also referred to the new genus. Vectipithex has a geographic distribution in north and west Europe and a stratigraphic distribution from the early Middle Eocene (Lutetian) to latest Eocene (late Priabonian). Cladistic analysis shows that Vectipithex is sister group to a clade composed of Microchoerus, Necrolemur, Nannopithex filholi, and ‘N.’ zuccolae and that poorly known ‘N.’ humilidens is sister group to those two. The Microchoerinae are shown to have undergone a phase of rapid diversification in the first few million years of the Eocene.