Peter A. Hochuli

Smithian-Spathian boundary event: Evidence for global climatic change in the wake of the end-Permian biotic crisis

One of the most important carbon cycle perturbations following the end-Permian mass extinction event straddles the Smithian-Spathian boundary (SSB) (Olenekian, Early Triassic). This anomaly is characterized by a prominent positive carbon isotope excursion known from Tethyan marine rocks. Its global signifi cance is established here by a new high paleolatitude record (Spitsbergen). Paleontological evidence, such as Boreal palynological data (Barents Sea, Norway) and global patterns of ammonoid distribution, indicates a synchronous major change in terrestrial and marine ecosystems near the SSB. The reestablishment of highly diverse plant ecosystems, including the rise of woody gymnosperms and decline of the formerly dominating lycopods, is interpreted as an effect of a major climate change. This hypothesis is supported by modeling of ammonoid paleobiogeography, the distribution patterns of which are interpreted as a proxy for sea surface temperatures (SST). The latest Smithian thus appears to have been a time of a warm and equable climate as expressed by an almost fl at pole to equator SST gradient. In contrast, the steep Spathian SST gradient suggests latitudinally differentiated climatic conditions. We propose that this drastic climate change and the global carbon cycle perturbation were triggered by a massive end-Smithian CO2 injection. The SSB event could therefore represent one of the causes for stepwise and delayed recovery of marine and terrestrial biotas in the wake of the end-Permian biotic crisis.

A volcanic CO2 pulse triggered the Cretaceous Oceanic Anoxic Event 1a and a biocalcification crisis

The Aptian Oceanic Anoxic Event 1a (OAE1a, ca.120 Ma ago) is one of the most prominent of a series of geologically brief intervals in the Cretaceous characterized by the deposition of organic carbon–rich sediments. OAEs reflect major perturbations in the global carbon cycle evidenced by sedimentary carbon isotope records. However, the triggering mechanisms for OAEs remain controversial. Here we present a bulk-rock and molecular (marine and terrestrial bio-markers) C isotope record at unprecedented time resolution, from the Cismon section of northern Italy, that shows that OAE1a conditions were reached over a period of several thousands of years through a stepwise perturbation of the carbon cycle. The documented sequence of events is most compatible with a trigger associated with increased CO 2 emissions, possibly leading to a doubling of p CO 2 , which in turn caused larger C isotope fractionation in marine and terrestrial organisms and a major biotic crisis in the calcareous nannoplankton. Our data also show that a release of isotopically light carbon from partial methane hydrate dissociation probably played a minor role in the OAE1a carbon cycle perturbation.

Episodes of high productivity and cooling in the early Aptian Alpine Tethys

A palynological and organic geochemical study of the early Aptian Selli unit (oceanic anoxic event 1a) of the western Tethys provides new information about paleoceanography and documents pronounced changes in the regional climate pattern. It also emphasizes the role of oxic degradation of organic matter, even at horizons with the highest rates of organic carbon (Corg) accumulation. The absence of anoxia indicates that enhanced primary production is the main reason for these organic carbon accumulations, and variations in preservation are related to pulses in primary production and nutrient recycling. Despite the high-productivity character of the Selli unit black shales, the δ13Ccarb values are not marked by a positive shift, but by an interval of unchanged values. Warm climatic conditions for deposition of the Selli unit are inferred from the presence of southern provenance pollen and low δ18O values. For the interval above the Selli unit, changes in the pollen spectrum and an increase in δ18O values reflect a hitherto undocumented late early Aptian cooler and more humid climate and/or a reorganization of the Tethyan oceanographic circulation pattern.

Climatic and biotic upheavals following the end-Permian mass extinction

The recovery from the end-Permian mass extinction was slow and prolonged. A temperature reconstruction shows that further biotic crises during the recovery were associated with extreme warmth. Recovery from the end-Permian mass extinction is frequently described as delayed1,2,3, with complex ecological communities typically not found in the fossil record until the Middle Triassic epoch. However, the taxonomic diversity of a number of marine groups, ranging from ammonoids to benthic foraminifera, peaked rapidly in the Early Triassic4,5,6,7,8,9,10. These variations in biodiversity occur amidst pronounced excursions in the carbon isotope record, which are compatible with episodes of massive CO2 outgassing from the Siberian Large Igneous Province4,11,12,13. Here we present a high-resolution Early Triassic temperature record based on the oxygen isotope composition of pristine apatite from fossil conodonts. Our reconstruction shows that the beginning of the Smithian substage of the Early Triassic was marked by a cooler climate, followed by an interval of warmth lasting until the Spathian substage boundary. Cooler conditions resumed in the Spathian. We find the greatest increases in taxonomic diversity during the cooler phases of the early Smithian and early Spathian. In contrast, a period of extreme warmth in the middle and late Smithian was associated with floral ecological change and high faunal taxonomic turnover in the ocean. We suggest that climate upheaval and carbon-cycle perturbations due to volcanic outgassing were important drivers of Early Triassic biotic recovery.

Timing of Early Cretaceous angiosperm diversification and possible links to major paleoenvironmental change

Palynological records from the Western Portuguese and Algarve basins (Portugal) provide new insights on the timing and pattern of the early diversification of angiosperms (flowering plants) and its relationship to global environmental perturbations during the late Early Cretaceous. Angiosperm pollen displays a stepwise increase in both diversity and relative abundance during the late Barremian to middle Albian interval (ca. 124–104 Ma), reflecting the incipient radiation of flowering plants in lower midlatitude floras. Our results provide new evidence for the age interpretation of the previously described angiosperm mesofossil floras and associated in situ pollen assemblages from the Western Portuguese basin, until now interpreted as Barremian or possibly Aptian in age. Biostratigraphic and sedimentologic evidence indicates a post-Aptian age for these assemblages, hence demonstrating a major radiation phase of angiosperms during the early Albian. Correlation of the angiosperm pollen record with data on global paleoenvironmental changes suggests a link between the rapid adaptive radiation of flowering plants and major climatic and oceanographic perturbations during the late Early Cretaceous.

Cyclic anoxic events in the Early Cretaceous Tethys Ocean

Cyclic black shale and nannofossil limestone units of the Lower Cretaceous Maiolica Formation of the southern Alps have been studied. From sedimentological data, the black shale units appear to have been formed in an euxinic basin that experienced periodic overturn and basinal aeration. The carbon-13 values of the calcareous nannofossils serve as indicators of paleocirculation. In the Barremian, the palynological spectra of the black shale units reveal a drastic alteration from a warm, dry to a cooler, humid climate. As a result, thermohaline circulation in the western Tethys Ocean was replaced by density stratification, and stagnation of the deeper basins followed.

Timing of early angiosperm radiation: recalibrating the classical succession

New stratigraphically controlled pollen data from the late Early Cretaceous of Portugal allow a recalibration of the classical continental succession of the Potomac Group (USA), which has long been used to demonstrate the gradual morphological changes of angiosperm leaves and pollen. The Portuguese record represents the best dated succession of angiosperm pollen assemblages of the interval between the late Barremian and the mid-Albian. Comparison of the angiosperm pollen record of the Potomac Group with the pollen assemblages from Portugal and with other independently dated records indicates that the distinct differences in the angiosperm pollen assemblages between the three formations of this group (the Patuxent Formation, Arundel Clay Formation and Patapsco Formation) are related to discontinuities. Our revised age model for the Potomac Group implies a major discontinuity between the Arundel Clay (of early Albian age), and the Patapsco Formation (of mid-Albian–early Cenomanian age). The Portuguese record reveals a so far undocumented radiation phase of angiosperms within the early Aptian to mid-Albian in which monocolpate angiosperm pollen of monocot and/or magnoliid affinity appear as a highly diversified group. These new findings imply that the onset of the radiation of monocots–magnoliids preceded the radiation of eudicots by at least 10 Ma.

Orbital control on carbon cycle and oceanography in the mid‐Cretaceous greenhouse

We established a new high-resolution carbonate carbon isotope record of the Albian interval of the Marne a Fucoidi Formation (Central Apennines, Italy), which was deposited on the southern margin of the western Tethys Ocean. Bulk carbonate sampled with 10–15 cm spacing was used for the construction of a continuous carbon isotope curve through the Albian stage. Spectral analyses reveal prominent 400 kyr cyclicity in the δ13C curve, which correlates with Milankovitch long eccentricity changes. Cycles occurring in our record resemble those observed in several Cenozoic δ13C records, suggesting that a link between orbital forcing and carbon cycling existed also under mid-Cretaceous greenhouse conditions. Based on comparisons with Cenozoic eccentricity-carbon cycle links we hypothesize that 400 kyr cycles in the mid-Cretaceous were related to a fluctuating monsoonal regime, coupled with an unstable oceanic structure, which made the oceanic carbon reservoir sensitive to orbital variations. In the Tethys these oceanographic conditions lasted until the Late Albian, and then were replaced by a more stable circulation mode, less sensitive to orbital forcing.

A boreal early cradle of Angiosperms? Angiosperm-like pollen from the Middle Triassic of the Barents Sea (Norway)

The origin of flowering plants is still a matter of dispute. Several lines of evidence suggest that their origin may go back to the Triassic. This paper reports on pollen grains with angiosperm-like morphologies from marine Middle Triassic sediments of the Boreal Realm (Norwegian Arctic, Barents Sea area). The morphology of these pollen grains is comparable to forms recorded from the Early Cretaceous, which are generally attributed to angiosperms. The new finds of angiosperm-like pollen are the earliest in the fossil record so far and show an astonishing high diversity. In contrast to other early records, they come from high palaeolatitudes with an inferred warm-temperate climate. The new finds suggest the presence of the first angiosperms during the Middle Triassic (242–227 Ma) or, alternatively, provide evidence for an as-yet unknown group of gymnosperms, possibly an extinct sister group of the flowering plants.

North Gondwanan floral elements in lower to middle cretaceous sediments of the Southern Alps (Southern Switzerland, Northern Italy)

Abstract Some early Cretaceous floral assemblages of the Southern Alps (southern Switzerland, northern Italy) contain typical North Gondwanan elements such as Dicheiropollis etruscus , “ Reticulatasporites ” jardinus, Reyrea sp., Elaterosporites sp., Elaterocolpites sp., Elateroplicites sp., and Galeocornea sp. This is the northernmost known occurrence of these sporomorphs. Their presence provides new data for the reconstruction of early Cretaceous floral provinces and palaeogeography. Comparison of sporomorph assemblages from different regions of the Northern Gondwana and the Southern Laurasia provinces gives evidence for climatic evolution during the middle Cretaceous.