ALBERT

Description: 〈span〉Knowledge of the age and compositional architecture of Archean cratonic lithosphere is critical for models of geodynamics and continental growth on early Earth, but can be difficult to unravel from the exposed geology. We report the occurrence of numerous 〉3.7 Ga zircon crystals in 3.45 Ga rhyolites of the eastern Pilbara Craton (Western Australia), which preserve evidence for an Eoarchean meta-igneous component in the deep Pilbara crust. This inherited zircon population shares similar and distinctive age and Hf-O isotope characteristics with the oldest gneissic components of the Yilgarn Craton ~500 km farther south, suggesting a common ca. 3.75 Ga felsic crustal nucleus to these two Archean granite-greenstone terranes. We infer a pivotal role for such ‘seeds’ in facilitating the growth and persistence of Archean continental lithosphere.〈/span〉

Description: U-Pb geochronology and Lu-Hf isotope analysis of detrital zircon from the mid-Cretaceous Winton and Mackunda Formations in the Eromanga Basin were employed to investigate regional provenance patterns in order to better understand the tectonic setting and paleogeography of eastern Australia during the late Mesozoic. A suite of Mesozoic-aged zircon populations recovered from these formations suggests that volcanism along the eastern margin of Australia was relatively continuous from the Triassic (252 Ma) to at least the mid-Cretaceous (ca. 92 Ma). Cretaceous-age zircon populations dominate the provenance record, and a distinct up-section younging trend in Cretaceous grain ages indicates that deposition was largely synchronous with ongoing volcanism to the east. Lu-Hf isotopic data suggest that these zircon populations were sourced from igneous rocks of a mixed juvenile and crustal source, similar to Lu-Hf isotopic systematics for eastern Australian zircons from Pennsylvanian–Permian igneous assemblages (307–252 Ma), for which an active convergent margin association is well established. An extensive Cretaceous volcanic terrain, now limited to the Whitsunday Igneous Association, was once located along the northeastern margin of Australia. Results from this study support the hypothesis that the Whitsunday igneous association was the main source of Cretaceous sediment to the Eromanga Basin, and likely for sediment transported across the continent southward and into the Ceduna Delta system offshore South Australia. The Whitsunday igneous association has been interpreted as a siliceous large igneous province associated with the onset of rifting in the region and linked to opening of the Tasman and Coral Seas. Yet, in this study, we document a relatively continuous Late Triassic to Late Cretaceous (240–92 Ma) age range for detrital zircons from the Mackunda and Winton Formations, consistent with relatively uninterrupted magmatic activity along the continental margin until ca. 92 Ma (earliest Turonian). Furthermore, zircon grains across this age spectrum exhibit dominantly positive to strongly positive Hf ( t ) values, between +4 and +12, consistent with values known for zircon suites from older magmatic arc rocks of eastern Australia. Although these data do not support a conclusive interpretation, they are consistent with an east Australian magmatic arc related to westward subduction of paleo-Pacific oceanic crust beneath eastern Australia enduring into the Cretaceous, as distinct from extensional siliceous large igneous province magmatism unrelated to subduction and generated by rupture of continental crust.

Description: The translation of climate signals into sediments is a complex and poorly understood process, and one that is likely to be inherently nonlinear. A great deal of debate concerns the mismatch between the amplitude spectrum of astronomical insolation forcing and the amplitude spectrum of climatic signals derived from geological archives. Many ancient shallow-water successions, such as peritidal carbonates, are widely suggested to show evidence for eccentricity-band forcing, despite eccentricity having a negligible effect on insolation. Instead, eccentricity manifests itself primarily as the amplitude modulator of precession, which dominates the insolation signal at low to mid latitudes. The appearance and dominance of eccentricity-band frequencies in stratigraphic data can arise, however, without the need to appeal to complex nonlinearities in the climate system or depositional environment. Instead, eccentricity frequencies will appear as a consequence of signal-driven nondeposition that results in a rectification of the precession-dominated insolation signal. Furthermore, it can be shown that this rectification effect is capable of inducing a signal-driven modulation of cycle thickness (i.e., a frequency modulation) that can lead to a similar range of cycle bundling patterns documented from ancient shallow-water successions. Numerical experiments using an astronomical insolation solution as an input signal emphasize how eccentricity can dominate climate records that have undergone only partial rectification, and that are stratigraphically complete at astronomical time scales of resolution. These experiments, supported by geological observations, thus describe a viable mechanism capable of explaining the eccentricity paradox in ancient shallow-water successions, and also emphasize how measures of stratigraphic completeness at astronomical time scales furnish no clue as to the likely fidelity of the actual climate forcing records or their spectra.

Description: Defining the time scale of abrupt events in the stratigraphic record is a primary goal of high-resolution paleoclimate analysis. A significant hurdle in this endeavor is that abrupt, i.e., millennial and submillennial, events in deep time can rarely be temporally constrained accurately owing to the typical absence of high-precision age control at the scale of the events. Instead, the duration of abrupt events is commonly estimated via the linear partitioning of time between age control points (e.g., defined using astronomical cycles or radiometric dates) that bracket the event and span longer time intervals. The flaw with this approach is that sedimentation is an unsteady process and does not proceed linearly with time. Here a numerical model, parameterized by geologic data, is used to quantify theoretical time-scale uncertainties that result from unsteady sedimentation. This work demonstrates that the duration of assumed millennial events estimated via a linear partitioning approach may be significantly in error, even in complete, astronomically calibrated and unbioturbated successions best suited to the study of abrupt paleoclimate change. The uncertainties established in this study are largely a function of the precise statistical properties of the sedimentation process, properties that are difficult to constrain empirically, particularly over short time spans. Nevertheless, this study illustrates how unsteady sedimentation sets an important limit on the attainable temporal resolution of the stratigraphic record, with consequent implications for defining accurately the rates and durations of rapid events in Earth history.

Description: Changes in the possible behavior of El Niño–Southern Oscillation (ENSO) with global warming have provoked interest in records of ENSO from past “greenhouse” climate states. The latest Cretaceous laminated Marca Shale of California permits a seasonal-scale reconstruction of water column flux events and hence interannual paleoclimate variability. The annual flux cycle resembles that of the modern Gulf of California with diatoms characteristic of spring upwelling blooms followed by silt and clay, and is consistent with the existence of a paleo–North American Monsoon that brought input of terrigenous sediment during summer storms and precipitation runoff. Variation is also indicated in the extent of water column oxygenation by differences in lamina preservation. Time series analysis of interannual variability in terrigenous sediment and diatom flux and in the degree of bioturbation indicates strong periodicities in the quasi-biennial (2.1–2.8 yr) and low-frequency (4.1–6.3 yr) bands both characteristic of ENSO forcing, as well as decadal frequencies. This evidence for robust Late Cretaceous ENSO variability does not support the theory of a “permanent El Niño,” in the sense of a continual El Niño–like state, in periods of warmer climate.

Description: 〈span〉〈div〉Abstract〈/div〉A melt contamination model is presented to explain the formation of “I-type” granitic rocks, based on studies of migmatitic mafic granulites from the Hidaka Metamorphic Belt (Japan). Analysis of apatite and zircon from leucosomes and mesosomes of the migmatites reveals grain-scale Nd and Hf isotope heterogeneities of 〉10 epsilon units, inconsistent with closed-system anatexis. We interpret this marked isotopic variability to instead reflect hybridization between metasedimentary-derived partial melts and interlayered mafic granulite horizons during extraction of silicic melt from the lower crust. This open-system melt-rock interaction induces local isotopic modification of the mafic granulites and shifts the Hf isotope signature of the anatectic melt to more radiogenic values, similar to those of hornblende-bearing (I-type) granitic rocks emplaced into the Hidaka sequence at higher crustal levels. This study shows that the generation of broadly I-type granitic magmas does not require extreme temperatures to extensively melt meta-igneous rocks, nor is the direct input of mantle magma essential. Mineral-scale isotopic heterogeneities in such magmas reflect derivation from contrasting crustal sources and the rate and mechanism of magma transfer through the granulitic lower crust.〈/span〉

Description: Constraining past fluctuations in global temperatures is central to our understanding of the Earth’s climatic evolution. Marine proxies dominate records of past temperature reconstructions, whereas our understanding of continental climate is relatively poor, particularly in high-latitude areas such as Antarctica. The recently developed MBT/CBT (methylation index of branched tetraethers/cyclization ratio of branched tetraethers) paleothermometer offers an opportunity to quantify ancient continental climates at temporal resolutions typically not afforded by terrestrial macrofloral proxies. Here, we have extended the application of the MBT/CBT proxy into the Cretaceous by presenting paleotemperatures through an expanded sedimentary succession from Seymour Island, Antarctica, spanning the latest Maastrichtian and Paleocene. Our data indicate the existence of a relatively stable, persistently cool temperate climate on the Antarctic Peninsula across the Cretaceous-Paleogene boundary. These new data help elucidate the climatic evolution of Antarctica across one of the Earth’s most pronounced biotic reorganizations at the Cretaceous-Paleogene boundary, prior to major ice-sheet development in the late Paleogene. Our work emphasizes the likely existence of temporal and/or spatial heterogeneities in climate of the southern high latitudes during the early Paleogene.

Description: Finely laminated Middle Eocene sediments from the central Arctic contain high abundances of the delicate, sea ice–dwelling fossil diatoms Synedropsis spp. and sea ice–rafted debris (sea ice–IRD), establishing an offshore seasonal sea ice regime ca. 47 Ma. Synedropsis spp. co-occur with other diatom taxa and microfossils requiring open water. This strongly indicates seasonality; nonetheless, seasonal reconstruction of the flux cycle cannot be resolved by standard bulk-sediment analysis, which destroys sedimentary fabrics and averages data within samples. Here we resolve and reconstruct seasonal-scale flux events from these sediments using backscattered electron imagery (BSEI) of resin-embedded sediment, a nondestructive technique that preserves the integrity of sedimentary microfabrics, thus revealing discrete productivity-flux events at ultrahigh (e.g., 〈30 μm) resolution. Seasonality is expressed at the submillimeter scale by successions of discrete mono-specific laminae and micro-lenses of Synedropsis spp., terrigenous material (sea ice–IRD), and open-water taxa, indicating that first-year ice existed in the central Arctic. Further, BSEI reveals millimeter-scale alternation of bundles of laminae and microlenses of two distinct types: one characterized by Synedropsis spp. and terrigenous material, the other by mainly open-water taxa and little terrigenous material. The sedimentation rate and preliminary assessment of annual cycles indicate suborbital variability on the order of multi-decadal to centennial duration; we argue that this reflects variations in the sea ice–season length.

Description: Carbon isotope and palynological analysis of the fine-grained organic carbon–rich lacustrine sediments that filled the Cretaceous-Paleogene (K-Pg) boundary–age Boltysh impact crater (Ukraine) preserve a uniquely complete and detailed record of a negative carbon isotope excursion in an expanded section of the early Danian that we estimate lasted as long as ~340 k.y. Palynological assemblages recovered through the excursion reflect the increasing dominance of thermophylic Normapolles species, indicating an increasingly warm and dry climate, while those recovered below and above the excursion reflect a cooler and wetter climate. The record of a transient warming event (hyperthermal) in the early Danian at Boltysh has strong similarities with the Dan-C2 hyperthermal event recorded in marine sediments in Tethys and the Atlantic Ocean, and suggests that there were profound environmental changes occurring on a global scale shortly after the K-Pg boundary.