Stratigraphic Research Articles

Archaeomagnetic Dating of Three Furnaces inside the Middle Age Settlement of San Genesio (San Miniato, Pisa, Italy)

Archaeomagnetic dating using full geomagnetic vector was performed on three furnaces cropping out at San Genesio archaeological zone, an ancient settlement located in the Arno River plain, near San Miniato (Pisa). The first evidence of human presence in this area dates back to the period between the VI century BCE and 1248 CE, when the village of San Genesio was destroyed by the inhabitants of the nearby castle of San Miniato. Three burned structures were located at different stratigraphic levels. The SGEN01 represents a kiln to produce pottery. The SGEN02 is probably a furnace for domestic use, while the SGEN03 is interpreted as a metallurgic kiln due to the presence of some hematite fragments possibly coming from Elba Island. Both mean paleodirections and absolute intensity were compared with the global geomagnetic model SCHA.DIF4K (Pavón-Carrasco et al., 2021) for Europe. The obtained age intervals at the 65% probability are 846-911 CE for SGEN01, 696-799 CE for SGEN02, and 623-644 CE for SGEN03. These new absolute dates agree well with their archaeological/stratigraphic position and with the history of the archaeological place.

Orbital-scale cyclicity and organic matter production in a Permian shallow mixed carbonate-siliciclastic system: Paraná Basin, Brazil

Mixed deposition in shallow seas is highly sensitive to climate variations and sea-level changes, which are the primary controls of high-frequency cycles in the stratigraphic record. The widespread Lower Permian mixed carbonate-siliciclastic succession of the Paraná Basin is composed of rhythmic organic-rich shale interbedded with dolostone. The strata are organized into several orders of magnitude of superimposed asymmetrical and symmetrical facies stacking patterns that developed in a broad epeiric sea, which extended over 106 km2 of Permian Gondwanan land. Detailed sedimentological and stratal stacking analysis allow the hierarchical organization to be identified in large-scale, medium-scale, small-scale, and elementary stacking patterns. The high-frequency depositional cycles compose four 3rd-order sequences. They comprise gray to black shale, marls, and peloidal to micritic dolostone. Due to the shallow water setting, the minimal available accommodation is responsible for the reduced cycle thickness and the overall mixed succession. The rhythmic deposits host highly productive hydrocarbon source rocks. Two widespread, thicker intervals of bituminous shale that develop during 3rd-order sequence transgression events are highly organic-rich oil shale (27 wt%). In addition, thin black shale laminae, which are interbedded with dolostone in high-frequency stacking patterns, are also rich in organic carbon (2–8 wt%). The alternation of high carbonate content and organic matter production was controlled by wet‒dry seasons and was related to total solar radiation maxima and minima. The total solar radiation maxima are translated into elevated carbonate precipitation due to shallower and hypersaline conditions. Total solar radiation minima induce freshwater discharge in shallow and hypersaline environments during wet stages, resulting in water column stratification. The occurrence of kerogen types III and IV, a high gammacerane index, variable Pr/Ph ratios, and C35 homohopane indices indicate this condition. In addition, the input of nutrient-rich terrigenous sediments favor algal blooms, enhancing anoxia in the bottom water, which is responsible for the organic-rich shale facies, as indicated by kerogen types I and II, high hydrogen index values, high C27/C29 sterane ratios, and higher β-carotane values. Based on the hierarchical organization and previous U–Pb SHRIMP zircon ages of volcanic ash layers, it is possible to correlate the observed succession with the global sea-level curve. A time span of 8.0 Ma is assessed for the Irati Sea, with an average of 2.7 Ma for the large-scale cycles. Furthermore, the high-frequency patterns that are compatible with the precession and eccentricity of the short (±100 ka) and major (±400 ka) periodicities of the Earth's orbit are estimated to be 26.6 ka, 135.5 ka, and 400 ka. The high-frequency cycles of organic-rich shales and carbonate that are deposited over thousands of years under long-term sea-level changes and are correlated along the basin suggest climatic control driven by the Earth's orbital perturbation, which is possible in the Milankovitch frequency band.

Disentangling tectonic and eustatic controls on forearc basin stratigraphy, Talara Basin, Peru

The controls on basin-fill stratigraphy such as sediment supply, climate, eustasy and tectonics are well understood in many basin types, but there are few examples documented from forearc basins. Traditional evolutionary models for forearc basins emphasise the importance of accretionary processes under a largely contractional tectonic regime. The Talara Basin of northwestern Peru preserves a lower Eocene stratigraphic record of dominantly extensional collapse tectonics punctuated by periods of uplift, driven by variability of the subduction process. The margin is dominated by subduction erosion and only minor accretion processes. In this geodynamic setting, we present for the first time a sedimentological and sequence stratigraphic model for the 350 m-thick fluvio-deltaic succession of the Pariñas Formation. Three depositional sequences are stacked into a highstand sequence set comprising proximal fluvial strata that correlate down dip into marginal marine and marine deposits. Syn-sedimentary normal faulting related to episodes of subduction erosion restricted much of the Pariñas to hanging-wall locations subject to minor-scale transgressions. The succession evolved from shelf-confined clinoforms to later basin margin scale clinoforms, related to long-term sea-level rise. The interaction of eustasy, sediment supply and episodes of subduction erosion are integrated into an evolutionary model that may have application in other non-accretionary forearc basins worldwide. Although the model shares characteristics with extensional models for rift basins, the episodic subduction-driven uplift events add another component of relative sea-level complexity.

Congregation leaving the Reformed Church in Nuenen by Vincent van Gogh: a combined multi-instrumental approach to analyse the painting’s stratigraphy in support of varnish removal

Vincent van Gogh’s painting Congregation Leaving the Reformed Church in Nuenen from the collection of the Van Gogh Museum in Amsterdam was executed in 1884 and partially repainted by the artist in 1885. The painting was restored in 1961, however, the details of this treatment were not documented. After being stolen from the museum in 2002 and finally recovered in 2016, the Church was subjected to an extensive technical examination campaign which started in 2017. The aims were to: characterise the stratigraphy of both initial and later paint layers (including identification of the painting materials used by Van Gogh), evaluate the condition of the painting and assess the feasibility of the desired restoration treatment. Portable X-ray fluorescence spectrometry (XRF) was performed to non-invasively identify elements related to pigments in the paint layers of the two painting campaigns. To further identify constituent materials and comprehend the painting’s complex stratigraphy, a single paint sample was collected and embedded in resin for analysis by means of Optical Microscopy, Scanning Electron Microscopy with Energy Dispersive X-ray spectrometry (SEM-EDS) and Fourier Transform Infrared spectrometry - Attenuated Total Reflectance (FTIR-ATR). Additional non-invasive measurements were performed in a MOLAB campaign in 2018 by two complementary and portable analytical techniques: Optical Coherence Tomography (OCT) and reflection FTIR spectroscopy were used to gain further insight into the painting’s stratigraphy and identify surface layers across various regions of the painting. The presence of an original varnish under the paint from 1885 (and therefore likely applied by Van Gogh himself) was revealed by OCT. It was characterised as being protein based by FTIR-ATR and reflection FTIR spectroscopy. Based on the knowledge on the artist’s varnishing practice, it could be concluded that this most likely concerns an egg white varnish for the first time found in an early work by Van Gogh. The upper varnish layer, however, was identified as an alkyd resin applied during the aforementioned 1961 treatment. The combined use of FTIR and OCT enabled non-invasive in situ assessment of solvent cleaning procedures aimed at the selective removal of the 1961 restoration varnish with the preservation of Van Gogh’s original varnish. Specifically, OCT and FTIR analyses were carried out before, during and after each cleaning test to carefully assess the condition of the painted surface and that of the original varnish. The results of the cleaning tests aided in fine-tuning the procedure of varnish removal during the restoration process.

Interpreting the mammal deposits of Cloggs Cave (SEAustralia),GunaiKurnaiAboriginal Country, through community‐led partnership research

AbstractPalaeontological animal bone deposits are rarely investigated through research partnerships where the local First Nations communities have a defining hand in both the research questions asked and the research processes. Here we report research undertaken through such a partnership approach at the iconic archaeological site of Cloggs Cave (GunaiKurnai Country, East Gippsland), in the southern foothills of SE Australia's Great Dividing Range.A new excavation was combined with detailed chronometric dating, high‐resolution 3D mapping and geomorphological studies. This allowed interpretation of a sequence of stratigraphic layers spanning from a lowermost excavated mixed layer dated to between 25,640 and 48,470 cal BP, to a dense set of uppermost, ash layers dated to between 1460 and 3360 cal BP. This long and well‐dated chronostratigraphic sequence enabled temporal trends in the abundant small mammal remains to be examined.The fossil assemblage consists of at least 31 taxa of mammals which change in proportions through time. Despite clear evidence that the Old Ancestors repeatedly carried vegetation into the cave to fuel cool fires (no visible vegetation grows in Cloggs Cave), we observed little to no evidence of cooking fires or calcined bone, suggesting that people had little involvement with the accumulation of the faunal remains. Small mammal bones were most likely deposited in the cave by large disc‐faced owls,Tyto novaehollandae(Masked Owl) orTyto tenebricosa(Sooty Owl).Despite being well dated and largely undisturbed, the Cloggs Cave assemblage does not appear to track known Late Quaternary environmental change. Instead, the complex geomorphology of the area fostered a vegetation mosaic that supported mammals with divergent habitat preferences. The faunal deposit suggests a local ancestral landscape characterised by a resilient mosaic of habitats that persisted over thousands of years, signalling that the Old Ancestors burned landscape fires to encourage and manage patches of different vegetation types and ages within and through periods of climate change.Read the freePlain Language Summaryfor this article on the Journal blog.

Local-scale spatial variability in firn properties in Southwest Greenland

The increasing mass loss from the Greenland ice sheet is becoming growingly important to present-day global sea level rise. Meltwater export to the ocean may be amplified by near-surface ice layers in firn (i.e., snow that has survived at least one melt season) which prevent infiltration. Such ice layers have been documented at various sites in southwest Greenland, but little is known about the local-scale (less than 1 km) variability in firn and ice properties. Here we investigate the local variability in firn density and ice content (ice layer thickness fraction) by analyzing 45 shallow firn cores retrieved from 15 sites in southwest Greenland between 2012 and 2019. The core dataset contains groups of cores that were collected at the same site (1–632 m apart from each other) and in the same year (here referred to as same site/year cores). The two-sided Mann-Whitney U-tests suggest that most same site/year cores do not have statistically significant differences in median density or ice content over the upper 0–4.5, 4.5–9, and 9–13.5 m sections below the winter snow layer. That said, the same site/year cores still exhibit substantial density and ice content differences as well as variable ice layer stratigraphy (especially the distribution of centimeters-thick ice lenses) not explainable by measurement uncertainties. The local-scale variability in density and ice content (calculated from the same site/year cores) over the three 4.5-m sections is 4% and 19–29%, respectively. The variability is comparable to the regional gradient in our study area within an elevation difference of up to ∼200 m. The local variability is non-negligible particularly at mid- to high-elevation sites (>2,000 m a.s.l.), where firn structure changes such as ice layer formation are more recent phenomena compared to lower elevations. This study confirms that single cores can provide representative bulk statistics such as mean density and ice content of one site. However, these mean values mask substantial local variability which can compromise regional comparisons. Therefore, collection of same site/year cores and consideration of local variability are necessary for an in-depth analysis of regional variability.

STRONTIUM ISOTOPE STRATIGRAPHY REVEALS 100 KY-SCALE CONDENSATION, BEVELING, AND INTERNAL SHINGLING OF TRANSGRESSIVE SHELL BEDS IN THE MARYLAND MIOCENE

Abstract Condensed transgressive shell beds, rich in paleobiological information, are common in the Phanerozoic stratigraphic record, but their interpretation is complicated by the uncertain amount of time that each deposit represents. Miocene strata exposed in the Calvert Cliffs (Maryland, USA) are known for a series of regionally extensive, densely packed, meters-thick shell beds that serve as global exemplars of shallow-water condensation during marine transgression and onlap. Applying Sr isotope stratigraphy to calcitic scallops from the base and top of the oldest of these beds (Shattuck Zone 10), we demonstrate that its most densely fossiliferous part accumulated over the interval of 16.60–15.95 Ma, reflecting approximately 0.65 ±0.20 Myr of skeletal accumulation within the Miocene Climatic Optimum, with a likely 0.15 Myr-scale of time averaging per each firmground-bounded subunit. Both of these estimates are an order of magnitude longer than previous best estimates based on radiocarbon-dated Holocene analogs. Sr isotopes confirm subtle low-angle erosional beveling of the main body of Zone 10, truncating 0.3-Myr of record in updip sections, and demonstrate that a down-dip wedge of less shelly sand is an entirely younger (by ∼ 0.5 Myr) interleaved body. This condensation, beveling, and inter-shingling within Zone 10 quantified here sets a precedent for the magnitude of lateral and vertical temporal variability within condensed transgressive deposits, relevant to paleobiologic and other geohistorical analysis, and justifies the interpretation of comparably complex temporal fabrics based upon similar physical stratigraphic features found elsewhere in this and other very thin stratigraphic records.

Shallow gas and gas hydrate accumulations influenced by magmatic complexes in the Pearl River Mouth Basin, South China Sea

We analyse occurrences of gas hydrates and free gases above a 1200 km2 complex of igneous volcanoes and sills using newly acquired three-dimensional (3D) seismic data in the Pearl River Mouth Basin of the South China Sea. In strata above volcano summits, we recognize three enhanced reflection layers at different stratigraphic levels, including discontinuous Bottom Simulating Reflectors (BSRs) and pluming BSRs. We also show that the lateral tips of eight sills, which at times appear to terminate against supra-sill faults, underlie gas chimneys linked to gas hydrate and free gas in shallower strata. These supra-sill faults and gas chimneys, as well as supra-volcano faults, provided pathways for upward fluid (gas) migration from deep sources. Given the spatial relationship between volcanoes, sills, free gas, and gas hydrates, we suggest that the igneous structures focused fluid migration into overlying faults and gas chimneys. We also explore whether higher geothermal gradient at the summits of the volcanoes caused the base of gas hydrate stability zone to be shallower, forming a pluming BSR. Our study highlights how volcanic systems can influence the formation and distribution of later gas hydrate and free gas.

PLASTER CHARACTERISTICS OF BYZANTINE WALL PAINTINGS IN WESTERN ANATOLIA

In this study, the execution technique and material characteristics of the plaster layers of Byzantine wall paintings from three archaeological sites (Anaia, Olympos and Aigai) from western Anatolia were examined for the purpose of their conservation. Throughout the study, mineralogical and chemical composition of the plaster layers were determined with a scanning electron microscope coupled with an X-ray energy dispersive system (SEM-EDS) and X-ray diffraction analyses (XRD). Reflected light microscopy (RLM) and SEM studies conducted on polished cross-sections of samples revealed the stratigraphy and microstructural properties of the plaster layers of the Anaia Church (4th–12th centuries), Başpınar Church (13th century) in Olympos and a Byzantine settling and the Chapel (13th century) in Aigai. Throughout the research, pure lime and aggregates in the plasters of Aigai, as well as magnesium-rich lime and straw in the plasters of Anaia and Başpınar were determined. The plasters from the Anaia and Başpınar Churches exhibit characteristics similar to the samples from the post-iconoclastic period due to a similar raw-material use. In the samples from Aigai, plastering techniques such as marmorino, intonachino and cocciopesto, indicating earlier periods, were observed. As a result of this study, it was concluded that the number and characteristics of the plaster layers from the paintings had been applied intentionally in accordance with the technique of the painting. Plaster characteristics of the wall paintings in western Anatolia from the Byzantine period were found to be similar to the ones in İstanbul, the Balkans, Crete and Cyprus. The results of this study will guide the conservation efforts of the wall paintings.

Disentangling microstructure and environmental conditions in high-altitude Andean microbialite systems (Catamarca, Argentine Puna).

The study of microbialites development is a key tool to understand environmental pathways during deposition. We provide a detailed analysis of modern Central Andean microbialites from high-altitude lakes. The stratigraphic record of Turquesa Lake shows a significant short-term recolonization by microbialite-producing microorganisms during environmental stress. Far from a crisis paradigm, the coasts and paleocoasts of Turquesa lake exhibit three microbialitic buildups formed along different stages, providing a good study case of biological resilience of these systems in harsh environments. The MI and MII microbialite buildups occupied two paleocoasts. Both are composed of oncoids with micritic to microsparitic textures. Morphological, textural and mineralogical similarities between the two buildups suggest that they were formed at different times, but under very similar environmental conditions. The microorganisms that produced the microbialitic buildup MIII are currently colonizing the coast of this lake. The previous oncoid morphology change to a parallel micritic-spartic lamination. This remarkable changes in the microstructure can be explained by an important environmental change caused by the isolation of the Peinado Lake, and a subsequently microorganism adaptation. This microbialite structures can be proposed as an interesting modern analogue for environmental changes along the geological record.

Tectonic Subsidence Modeling of Diachronous Transition From Backarc to Retroarc Basin Development and Uplift During Cordilleran Orogenesis, Patagonian‐Fuegian Andes

AbstractBackstripped tectonic basin subsidence histories are critical for interpreting phases of lithospheric deformation and paleoenvironmental change from the stratigraphic record. This study presents new subsidence modeling of the Rocas Verdes Backarc Basin (RVB) and Magallanes‐Austral retroarc foreland basin (MAB) of southernmost South America to evaluate along‐strike changes in tectonic subsidence related to the Late Jurassic through Miocene history of the Southern Andes. We compiled composite stratigraphic sections for seven basin localities that span 47°–54°S from published sedimentological records of paleoenvironment, paleobathymetry, and geochronology. Modeling results resolve regional trends in basin tectonic subsidence, uplift, and sedimentation rate that influenced the depositional environment during five broad phases of RVB‐MAB development: (a) Late Jurassic tectonic subsidence and basin deepening associated with rift‐related backarc extension that postdated regional diachronous rift‐related magmatism. (b) Southward younging of Early to Late Cretaceous pronounced acceleration in tectonic subsidence interpreted as the initiation of flexural loading and development of the MAB foreland basin system. (c) Late Cretaceous (ca. 85–70 Ma) tectonic uplift within the central foredeep ∼49° to 52°S, coeval with a shift from slope to shelf deposition at these latitudes. (d) A protracted period of low‐magnitude basin uplift and relative tectonic quiescence during the Paleogene, with the exception of southernmost localities; and (e) Synchronous latest Oligocene‐early Miocene tectonic subsidence linked to basin deepening and transgression across the northern and central basin sectors. Backstripped tectonic subsidence analysis corroborates existing interpretations for orogenic development in the RVB‐MAB and sheds new light on complex polyphase basin histories where extension precedes convergence.

The eastern Kendeng Hills (Java, Indonesia) and the hominin-bearing beds of Mojokerto, a re-interpretation

The eastern Kendeng Hills (Java, Indonesia) expose a 1000 m thick series that is used as a stratigraphic standard, representing the emergence of eastern Java from the sea. The fluvial top is rich in vertebrate fossils and yielded the Mojokerto (Perning) hominin skullcap, which is regarded as the earliest evidence of Homo erectus on Java, with age estimates ranging between 1.9 and 1.49 Ma. The series is commonly regarded as an uninterrupted record of coastal progradation. However, recent studies show that the emergence of eastern Java has been a complex process, under influence of tectonism, volcanism, sea-level fluctuations and fluvial dynamics, leaving a fragmented depositional record that varies from site to site. This is at odds with the prevailing stratigraphic practice of long-distance correlations and questions the existing interpretations of the eastern Kendeng reference sections. Here we present the results of a fieldwork-based re-interpretation of this key stratigraphic record, which we identified as the fill of a previously unrecognized Plio-Pleistocene embayment, surrounded by elongate uplift zones. Clinoform-bedded sandstones relate to a stage of explosive, high-silica volcanism, supplying large volumes of ash. The embayment fill is incised and covered by fluvial deposits, which we relate to the Middle Pleistocene Brantas. The fluvial strata have a cyclic build-up, probably representing sea-level controlled stages of aggradation and degradation. Based on a reconstruction of fluvial cycles, we provisionally link the conglomerate bed in which the Mojokerto Homo erectus was found to MIS14 (∼550ka). We infer that the published radiometric ages derive from reworked volcanic clasts that make up this incisive fluvial lag and are not representative for the age of deposition. Our study places the eastern Kendeng series in a new landscape context and changes our view of the timing of hominin migration to Java.

Neural net detection of seismic features related to gas hydrates and free gas accumulations on the northern U.S. Atlantic margin

Bottom-simulating reflections (BSRs) that sometimes mark the base of the gas hydrate stability zone in marine sediments are often identified based on the reverse polarity reflections that cut across stratigraphic layering in seismic amplitude data. On the northern U.S. Atlantic margin (USAM) between Cape Hatteras and Hudson Canyon, legacy seismic data have revealed pronounced BSRs south of the deepwater extension of Hudson Canyon and more subtle ones from offshore Delaware south to Cape Hatteras, where the reflections sometimes follow stratigraphic layering. Using high-resolution seismic data acquired during the 2018 Mid-Atlantic Resource Imaging Experiment and a supervised neural net, we identify seismic features associated with gas hydrates and/or the top of gas between Hudson Canyon and Cape Hatteras. Using seismic attributes especially sensitive to the presence of gas, we train a neural network algorithm on seismic data from an area with strong BSRs and then apply the model to the rest of the data set. The results indicate that gas hydrate and/or shallow free gas are significantly more widespread on the northern part of the USAM than previously known. Seismic indicators of gas extend landward from the 2000 m isobath to the upper continental slope in sectors with (offshore Virginia) and, to a lesser extent, without (offshore New Jersey) pervasive upper slope methane seeps. Higher sand content and intermediate sediment thickness, factors related to the container size and gas charge in a petroleum systems framework, are associated with more robust gas indicators.

Shape-constrained geophysical inversion

Two long-standing challenges for geophysical inverse problems are nonuniqueness (especially in the multiparameter setting) and the slow convergence of gradient-based optimization methods. We introduce shape-based inversion algorithms to address these challenges. The shape-based algorithms determine, based on the available geophysical data, the underlying subsurface structures (e.g., stratigraphic layers) and the spatially varying unknown physical parameters (e.g., velocity and density). Inverting simultaneously for geologic structures and physical parameters eliminates degeneracies between parameters and also provides a multiscale framework for decomposing the inverse problem. First, we provide a rigorous mathematical formulation of shape-based algorithms through diffuse interface approximations. Then, we demonstrate the applicability of these algorithms to a range of synthetic model problems in traveltime tomography, full-waveform inversion, gradiometry, and history matching of single-phase flow. These examples highlight the ability of shape-based inversion to mitigate nonuniqueness and improve the rate of convergence to a physically realistic solution.

Carbonate platform drowning caught in the act: The sedimentology of Saya de Malha Bank (Indian Ocean)

AbstractMesophotic reefs, hardgrounds and current‐controlled pelagic to hemipelagic carbonates are facies marking carbonate platform drowning successions, irrespective of the factors controlling this evolution. A modern analogue of a carbonate platform in a state of drowning, where these facies occur has not been properly reported on to date. In the present study, the sedimentary environments of the Saya de Malha Bank are characterized using a multi‐disciplinary approach including sedimentology, hydroacoustics, seismics and oceanography. The Saya de Malha Bank edifice with a surface of 40 808 km2 is located in the tropical Indian Ocean and lies in a water depth of 8 to 300 m extending from the surrounding more than 2000 m deep ocean floor, with no reef reaching the sea surface. Mesophotic coral and red algal facies co‐exist with hemipelagic and bioclastic sands, together with a hardground. Ocean currents and internal waves are identified as major sedimentological controlling factors in the absence of elevated nutrient influx. Many features distributed along the present‐day Saya de Malha Bank were described from studies presenting fossil examples of carbonate platform drowning. The results herein can therefore be applied to other drowning examples, in some cases allowing for more accurate interpretation of the stratigraphic record.

Relative lake level reconstructions for glacial Lake Agassiz spanning the Herman to Campbell levels

Lake Agassiz was the largest North American pro-glacial lake during the last deglacial period, and the routing of the overflow is hypothesized to have affected thermohaline circulation and in turn, global climate. Here an existing glacial isostatic adjustment model is applied to Campbell beach strandline data to synthesize and correlate published radiocarbon- and OSL-dated lake level indicators in an effort to produce relative lake-level curves and a drainage history. The curves highlight where the water level history of Lake Agassiz is well understood, and where additional work is necessary. Many OSL-dated strandlines between 14.5 and 13.2 ka record meltwater routing through a southern outlet to the Gulf of Mexico during this time, and that lake levels fell due to sill incision and glacial isostatic adjustment (GIA). Oxygen isotope records from marine sediments tracking continental runoff, plus deglacial isochrones, suggest eastern outlets to the Atlantic Ocean opened at around 13.2 ka, near the onset of the Younger Dryas (YD) stadial, but the evidence within the basin is scant. At some point during the YD, Lake Agassiz overflow shifted to a northwestern outlet into the Arctic Ocean, and the reconstructed lake levels track lake-level rise driven by uplift of the northwestern outlet, which eventually transferred water to the southern outlet to create the Campbell level of Lake Agassiz. The apparent shallow water depth (∼4 m) of overflow at the southern outlet relative to earlier phases suggests multiple active outlets at this time (northwestern, southern, and perhaps eastern). Oxygen isotope records from the Gulf of Mexico indicate overflow via the southern outlet occurred in two brief phases, likely explained by an oscillating ice margin that regulated overflow. Considerable work remains to assess the timing of deglaciation to understand when Lake Agassiz first drained east and north. Lacking are details of eastern drainage and any dated stratigraphic record of likely lake level fluctuations during the Younger Dryas.

Multidecadal delay between deglaciation and formation of a proglacial lake sediment record

Basal ages obtained from proglacial lake sediments are often used to constrain deglaciation histories, based on the assumption that fine-grained stratigraphic records start forming immediately after glacier retreat. Here, we test this assumption by studying the onset of sedimentation in Calluqueo Lake, Chilean Patagonia, which progressively deglaciated between the 1940s and the end of the 20th century. Although the glacier-proximal basin has been ice-free for at least three decades, it does not yet contain a fine-grained stratigraphic record, despite the modern sedimentation rate of ∼3 cm yr−1. By comparison, the distal basin contains a fine-grained stratigraphic record starting in 1997 ± 2 CE, i.e., 20–50 years after it was deglaciated. Based on these results, we show that several decades are required for ∼1 m of fine-grained sediments to accumulate between the coarse till material and sufficiently smoothen the uneven lake floor to start forming a fine-grained stratigraphic record. Although the exact timing depends on lake floor morphology and sediment accumulation rates, our results suggest that proglacial lake sediment records lack the first decades of sedimentation. This delay is mostly negligible when using radiocarbon ages from basal sediments to date glacier retreat since it falls within the range of radiocarbon uncertainties, though its importance increases throughout the Holocene. It is however significant for chronologies entirely based on varve-counting. Therefore, our results support the use of basal ages to establish deglaciation chronologies but they call for attention when using them to reconstruct Holocene ice retreat, especially with varve chronologies.

A biotic perspective on the Oligo-Miocene evolution of the Maldives carbonate platform from forward stratigraphic modelling (Indian Ocean)

Forward stratigraphic modelling is a fast-developing modelling approach, used to test conceptual models, and predict stratigraphic architecture and depositional facies from basin to reservoir scales. Published subsurface applications demonstrate its added value by integrating multidisciplinary data as well as geological concepts into its constraints. When applied to carbonate depositional systems, composed of multiple sediment factories, the co-operating and interdependent production mechanisms remain poorly studied. By applying the technique to a well-studied section of the Maldives carbonate platform, a specific model design—adapted to the geological age and setting, and constrained by available data—sheds light on the interaction of its carbonate producers. The results yield a naturalistic depositional facies distribution and offer insight in the changing relationship between biotic communities during the platform evolution. After calibration, the reference model unequivocally links the formerly proposed genetic model to the seismostratigraphic architecture. Furthermore, the results show how environmental changes (seemingly of secondary impact compared to changes in physical accommodation in the stratigraphic record) can induce substantial fluctuations in carbonate production rates of biotic communities, affect the ecological accommodation, and thus impact the platform architecture. Therefore, it is crucial to treat carbonate production rates during periods of environmental change as variables with associated uncertainties in a forward stratigraphic model setup.

Earth's Isostatic and Dynamic Topography—A Critical Perspective

AbstractEarth's topography arises from the linear superposition of isostatic and dynamic contributions. The isostatic contribution reflects the distribution of thickness and density of the crust overlying a static, non‐convecting mantle. We argue that isostatic topography should be limited to the crust, thereby delimiting all sources for dynamic topography below the Moho. Dynamic topography is the component of the topography produced by normal stresses acting on the Moho that deflect the isostatic topography away from crustal isostatic equilibrium largely as a consequence of mantle flow dynamics. These normal stresses arise from pressure variations and vertical gradients of the radial flow in the convecting mantle. The best estimate of dynamic topography is from the residual topography, which is the difference between observed topography and crustal isostatic topography. Dynamic and residual topography are the same. It is clear that thermal anomalies horizontally advected by plate motions would not exist if the mantle were not convecting, therefore their contribution to topography is inherently dynamic in origin. The global integral of dynamic topography that encompasses all non‐crustal buoyancy sources is demonstrated to be equal to zero. It follows that mantle convection cannot change the mean radius or mean elevation of the Earth. Since changes in ocean basin volume driven by changes in mean depth of the oceans are inherently part of dynamic topography, thereby requiring that continental elevations must also change, such that the global integral of these perturbations must also be equal to zero. This constraint has important implications for global long‐term sea level and the stratigraphic record, among other features of the Earth system impacted by changes in Earth's dynamic topography.

First record of shark microremains from the Lower Khartam Member, Khuff Formation: an Upper Permian origin of the genus Lissodus, or a new placement of the Permo/Triassic boundary in Saudi Arabia?

Shark fossil microremains belonging to the genus Lissodus are recovered for the first time from the Lower Khartam Member of the Khuff Formation in central Saudi Arabia. Previously published research assigned the Lower Khartam Member to the Upper Permian, implying that this is the oldest known record of the genus Lissodus. On the other hand, the stratigraphic occurrence of the recovered Lissodus lies slightly below the previously defined Permian/Triassic boundary in Saudi Arabia. Because Lissodus specimens are common from the Lower Triassic to the Upper Cretaceous, they may provide paleontological evidence for a revised placement of the Permian/Triassic boundary in Saudi Arabia. The Permian/Triassic boundary has been formerly placed at the contact between the Lower and Upper Khartam members of the Khuff Formation in central Saudi Arabia. Earlier studies relied on sequence stratigraphy and geochemistry as tools for identifying the Permian/Triassic boundary. This resulted in placing the boundary at different stratigraphic levels, reflecting the ongoing uncertainty about its actual position. In this study, two scenarios are discussed arguing either for a revised placement of the Permian/Triassic boundary, or a possible Late Permian origin for the genus Lissodus on the Gondwanan continent.