Angela M. Hessler

Stratigraphic record across a retroarc basin inversion: Rocas Verdes–Magallanes Basin, Patagonian Andes, Chile

The Mesozoic evolution of the Andean Cordillera of southern Patagonia is recorded in two formations that are now part of the fold-thrust belt: the Zapata Formation and the Punta Barrosa Formation. Extension in the Ultima Esperanza region began in the Late Jurassic with deposition of the marine volcaniclastic Tobifera Formation and eventually resulted in the full-fledged oceanic Rocas Verdes Basin. The Zapata Formation was deposited over a time span of ∼50 m.y. in an irregular basin ultimately bordered by an Early Cretaceous arc to the west. It is characterized by interbedded shale and siltstone mostly deposited in shallow water and, in areas of oceanic crust emplacement, also as deep-water hemipelagic deposits. Sediments of the Zapata Formation were derived initially from local oceanic upwarps and Tobifera highs and later from the ande-sitic cover of the juvenile arc and/or exhumed oceanic crust. The paucity of sandstone in the Zapata Formation in the Ultima Esperanza region indicates a highly irregular basin partitioned by prominent horsts, with sand deposition confined to sub-basins closer to the arc. Changes in depositional regimes and sediment dispersal patterns related to the onset of Andean contraction and formation of the Magallanes foreland basin are recorded by sediments of the overlying Punta Barrosa Formation. This formation records the evolution of a fold-thrust belt on the basis of the multimodal mineralogical and geochemical character of its sandstone and shale. The presence of an arc is indicated, but nearby Andean metamorphic terranes are more significantly represented in Punta Barrosa sediments. Stratigraphic details across the Zapata and Punta Barrosa Formations indicate that deformation and development of a proto–Andean Cordillera in southern South America was initiated in the late Mesozoic and involved conspicuous crustal shortening.

A lower limit for atmospheric carbon dioxide levels 3.2 billion years ago

The quantification of greenhouse gases present in the Archaean atmosphere is critical for understanding the evolution of atmospheric oxygen, surface temperatures and the conditions for life on early Earth. For instance, it has been argued that small changes in the balance between two potential greenhouse gases, carbon dioxide and methane, may have dictated the feedback cycle involving organic haze production and global cooling. Climate models have focused on carbon dioxide as the greenhouse gas responsible for maintaining above-freezing surface temperatures during a time of low solar luminosity. However, the analysis of 2.75-billion-year (Gyr)-old palaeosols—soil samples preserved in the geologic record—have recently provided an upper constraint on atmospheric carbon dioxide levels well below that required in most climate models to prevent the Earths surface from freezing. This finding prompted many to look towards methane as an additional greenhouse gas to satisfy climate models. Here we use model equilibrium reactions for weathering rinds on 3.2-Gyr-old river gravels to show that the presence of iron-rich carbonate relative to common clay minerals requires a minimum partial pressure of carbon dioxide several times higher than present-day values. Unless actual carbon dioxide levels were considerably greater than this, climate models predict that additional greenhouse gases would still need to have a role in maintaining above-freezing surface temperatures.

Petrogenesis and provenance of distal volcanic tuffs from the Permian–Triassic Karoo Basin, South Africa: A window into a dissected magmatic province

We present zircon rare earth element (REE) compositions integrated with U-Pb ages of zircon and whole-rock geochemistry from 29 volcanic tuffs preserved in the Karoo Supergroup, South Africa, to investigate the history of magmatism in southern Gondwana. Whole-rock compositions suggest a subduction-driven magmatic arc source for early (before 270 Ma) to middle Permian (270–260 Ma) Karoo tuffs. After ca. 265 Ma, the magmatic source of the volcanic deposits transitioned toward intraplate shallow-sourced magmatism. Zircon U-Pb ages and REE chemistry suggest that early to middle Permian magmas were oxidizing, U- and heavy (H) REE–enriched, melts; middle Permian to Triassic zircons record HREE-depleted, more reduced magmatism. Middle Permian to Triassic tuffs contain increasingly large volumes of zircon cargo derived from assimilated crustal material; therefore magmas may have been zircon undersaturated, resulting in less zircon growth and increased inheritance in late Permian to Triassic Gondwanan volcanics. Zircon U-Pb ages and zircon REE chemistry suggest a shift from arc magmatism in the early Permian to extensional magmatism by the late Permian, which may be associated with development of a backarc magmatic system adjacent to western Antarctica that predates known extensional volcanism elsewhere in Gondwana. Opening of the Southern Ocean in the Jurassic–Cretaceous paralleled this extensional feature, which may be related to reactivation of this Permian–Triassic backarc. This study demonstrates the potential of zircon U-Pb age and REE compositions from volcanic tuffs preserved in sedimentary strata to provide a more complete record of magmatism, when the magmatic province has been largely lost to active tectonism.

The ancestral Mississippi drainage archived in the late Wisconsin Mississippi deep-sea fan

The response of continental-scale drainage systems to short-term (i.e., millennial-scale) climate change is unknown but has wide implications for understanding climate feedbacks and terrestrial-marine fluxes. The late Wisconsin Mississippi River to deep-sea fan of North America was one of Earth’s largest sediment-routing networks during the most recent glacio-eustatic cycle. To understand late Pleistocene sediment production and dispersal related to the partly glaciated, ancestral Mississippi system, we sampled late Wisconsin deep-sea fan channel-fill and lobe deposits for detrital zircon U-Pb and (U-Th)/He double-dating analyses, from Deep Sea Drilling Project (Leg 96) cores and U.S. Geological Survey piston cores. Our results suggest a late Pleistocene glacial Mississippi system that forced a larger transfer of sediment from Cordilleran magmatic provinces and the Canadian Shield when compared to the modern drainage. This indicates a potentially more expansive and/or erosive ancestral Mississippi catchment, and the efficient dispersal of terrigenous sediment, nutrients, and solutes into the deep-sea via high-discharge meltwater and glacial-lake outbursts during ice retreat.

Late Pleistocene glacial transitions in North America altered major river drainages, as revealed by deep-sea sediment

Sediment eroded from continents during ice ages can be rapidly (<104 years) transferred via rivers to the deep-sea and preserved in submarine fans, becoming a viable record of landscape evolution. We applied chemical weathering proxies and zircon geo-thermo-chronometry to late Pleistocene sediment recovered from the deep-sea Mississippi fan, revealing interactions between the Laurentide ice sheet (LIS) and broader Mississippi–Missouri catchment between ca. 70,000 and 10,000 years ago (70 to 10 ka). Sediment contribution from the Missouri catchment to the Mississippi fan was low between 70 and 30 ka but roughly doubled after the Last Glacial Maximum (LGM). Therefore, pre-LGM glacial advance profoundly altered the vast Missouri drainage through ice dams and/or re-routing of the river, thereby controlling the transfer of continental debris and freshwater toward southern outlets.

Subduction zones and their hydrocarbon systems

Subduction zones are common tectonic features central to large-scale crustal and elemental cycling, and they are accompanied by basins often with thick sedimentary fill and structures suitable for hydrocarbon preservation. However, significant hydrocarbon production occurs in only a handful of subduction zone locations. Here we explore our current understanding of the controls on hydrocarbon systems associated with subduction zones, in terms of the strongly variable conditions inherent to this tectonic setting that either favor or limit petroleum production, and in the context of three case studies (Cook Inlet and Sacramento basins, USA; Talara basin, Peru). This review concentrates on continental rather than intra-oceanic subduction settings due to limited basin preservation and hydrocarbon prospectivity in the latter. Overall, the primary limitations on hydrocarbon potential in forearc and/or trenchslope basins are time-to-maturation (low geothermal gradients), reservoir quality, source-rock presence and quality, structural complexity, and depth to reservoir. The latter two conditions may explain why offshore exploration has been limited near subduction zones, even where onshore production is robust and/or hydrocarbon seeps are common. Prospectivity may increase with enhanced seismic imaging and offshore infrastructure in some locations, and with the economic development of unconventional resources such as gas hydrates in accretionary prisms or deep shale gas in forearc basins. In any case, the presence of hydrocarbon systems in subduction zones, whether prospective or not, is an important part of the cycling of carbon and other elements at active convergent margins.