Stephen T. Grimes

Understanding fossilization: Experimental pyritization of plants

The process of fossilization is poorly understood. However, it is central to our understanding of the evolution of life. It is unclear how plant tissues become fossilized, whether fossilization is selective to specific biopolymers, or whether original organic constituents survive. We have replicated the fossilization process in the laboratory by using both microbial and chemical approaches to pyritize plant debris. These results demonstrate that initial pyritization can be an extremely rapid process (within 80 days) and is driven by anaerobic bacterial-mediated decay. Initially, pyrite precipitates on and within plant cell walls and in the spaces between them. Further decay and infilling at all scales preserves broad cellular anatomy. The results have implications for fossilization in general and the fidelity of the taxonomic and biomolecular information preserved in fossils.

Fossil plants from the Eocene London Clay: the use of pyrite textures to determine the mechanism of pyritization

Pyritized twigs and roots from the Eocene London Clay of SE England were studied to gain a better understanding of the process of pyritization by investigating pyrite textures in relation to cell type and quality of preservation. Highly polished sections and fractured surfaces taken from 124 specimens were examined using optical microscope and SEM, the latter equipped to map pyrite and carbon. Pyrite textures include microcrystalline, framboidal, massive polycrystalline, and subhedral or euhedral forms. The highest fidelity of preservation is always associated with microcrystalline pyrite precipitation on wall surfaces with subsequent infilling of cells with framboids or polyhedra preventing compression during burial but contributing nothing to actual ultrastructural preservation. Ultrastructurally, parenchymatous cell walls are coalified, whereas microcrystalline pyrite plus coalified material were observed within lignified cell walls. In all, four stages of pyritization are documented. Observations are interpreted in the light of recent experiments on pyritization of living material and the chemistry of pyrite formation in anoxic environments involving an aqueous, and hence mobile, FeS cluster complex as a precursor. The complexity of the fossilization process is reflected in the presence of different textures in adjacent cells of the same tissue. This demonstrates the development of isolated chemical microenvironments as pH and Eh vary in response to decay, and mineralization and pyrite overgrowths within a cell indicate local microenvironmental changes through time.

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.

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.

Isotopic analysis of coexisting Late Jurassic fish otoliths and molluscs: Implications for upper-ocean water temperature estimates

Ophosphate of fi sh teeth are ABSTRACT The δ 18 O compositions of well-preserved Jurassic fi sh otoliths from Wootton Bassett, UK, provide upper-ocean paleotemperatures that are comparable with those derived from the iso- topic analysis of fitooth phosphates, providing independent scrutiny of such paleotempera- tures. δ 18 O otolith temperatures in excess of 30 °C also rival temperatures associated with the middle Cretaceous thermal maximum. The negative carbon isotopes of the otoliths may point to a freshwater infl uence and potentially migratory nature of the fi sh. However, given the large departures from equilibrium fractionation toward more negative carbon values reported from modern marine fi sh, we consider our temperature interpretations to be robust and representa- tive of the marine depositional environment. Depleted δ 13 C values, we believe, suggest that the otoliths examined in this study belong to fiwith high metabolic rates.

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 U–Th calcite isochron age from an active geothermal field in New Zealand

We report here the first U–Th disequilibrium age for a hydrothermal mineral from an active geothermal system in New Zealand. Vein calcite recovered from a depth of 389 m in Well Thm-1 at the Tauhara geothermal field has an age of 99±44 ka BP. This age was determined using a leachate–leachate isochron technique on four silicate containing sub-samples of calcite from a single vein. Although the error on this isochron age is considerable, it is significantly younger than the earlier estimated age of ∼200 ka BP for the onset of activity at the Tauhara system and probably records the date of brecciation and veining, which may be associated with volcanic activity at the adjacent dacitic Tauhara Volcanic Complex. These results demonstrate that hydrothermal vein calcite can now be dated directly, and opens the way for more detailed studies of the evolution of the New Zealand geothermal systems.

Coupling of marine and continental oxygen isotope records during the Eocene-Oligocene transition

While marine records of the Eocene-Oligocene transition indicate a generally coherent response to global cooling and the growth of continental ice on Antarctica, continental records indicate substantial spatial variability. Marine Eocene-Oligocene transition records are marked by an ∼+1.1‰ foraminiferal δ18O shift, but continental records rarely record the same geochemical signature, making both correlation and linking of causal mechanisms between marine and continental records challenging. Here, a new high-resolution continental δ18O record, derived from the freshwater gill-breathing gastropod Viviparus lentus , is presented from the Hampshire Basin, UK. The Solent Group records marine incursions and has an established magnetostratigraphy, making it possible to correlate the succession directly with marine records. The V. lentus δ18O record indicates a penecontemporaneous, higher-magnitude shift (>+1.4‰) than marine records, which reflects both cooling and a source moisture compositional shift consistent with the growth of Antarctic ice. When combined with “clumped” isotope measurements from the same succession, about half of the isotopic shift can be attributed to cooling and about half to source moisture change, proportions similar to marine foraminiferal records. Thus, the new record indicates strong hydrological cycle connections between marine and marginal continental environments during the Eocene-Oligocene transition not observed in continental interior records.