Exploring computed tomography in ichnological analysis of cores from modern marine sediments

Ichnological analysis is considered a very useful tool in several disciplines of Earth Sciences, including palaeoenvironmental studies and hydrocarbon exploration. Sediment cores provide excellent records, despite difficulties encountered during study runs due to specific core features. Previous studies using 2D images have proven the benefits of high-resolution image treatment in improving the visibility of ichnological features, but with limitations. 3D computed tomography (CT) techniques were applied to palaeoichnological studies in lithified cores and other disciplines of palaeontology to solve these limitations, but not used for ichnological studies in unconsolidated sediments due to the low density contrast between host sediment and trace fossils. In this study, a CT processing technique, previously tested in coral research, is applied to facilitate the characterisation of the ichnological signature of cores from modern marine soft sediments. This technique allows for the first time the isolation of burrows within these kinds of sediments and the differentiation of intervals based on burrow orientation. Data obtained from the technique are complemented with the ichnological information from conventional core description, thus providing a more complete characterisation of the trace fossil assemblage with additional ichnological properties such as burrow orientation and branching. This will improve palaeoenvironmental interpretations related to changes in energy or oxygenation, and the analysis of reservoir quality given the impact of burrows on porosity and permeability. Therefore, adopting CT to complement visual core description in the ichnological analysis of soft modern marine cores is a very informative approach.This work is supported by the Spanish Government [Project CGL2015-66835-P, Secretaría de Estado de I + D + I], Andalusian Government [Research Groups RNM-178 and RNM-276], and University of Granada [Scientific Excellence Unit UCE-2016-05]. The research of J.D. is financed by the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant Agreement No. 792314 (ICON-SE)

Atlantic water inflow to Labrador Sea and its interaction with ice sheet dynamics during the Holocene

The hydrodynamics of the Labrador Sea, controlled by the complex interplay of oceanographic, atmospheric and ice-sheet processes, play a crucial role for the Atlantic Meridional Overturning Circulation (AMOC). An improved understanding of the hydrodynamics and its forcing in the past could therefore hold a key to understanding its future behaviour. At present, there is a remarkable temporal mismatch, in that the largely microfossil-based reconstructions of Holocene Atlantic-water inflow/influence in the Labrador Sea and Baffin Bay appear to lag grain size-based current strength reconstructions from the adjacent North Atlantic by > 2ka. Here, we present the first current strength record from the West Greenland shelf off Nuuk to reconstruct Atlantic Water (AW)-inflow to the Labrador Sea via the West Greenland Current. Our data show that the Holocene AW-inflow into Labrador Sea is well aligned with the Holocene Speed Maximum documented in the North Atlantic (McCave and Andrews, 2019; Quat. Sci. Rev. 223), suggesting a close coupling with the AMOC. The observed lag between the microfossil-based records and the Holocene Speed Maximum can be explained when considering the presence of an extended meltwater lens that prevented the shoaling of the inflowing Atlantic waters. Once the meltwater discharge waned after the cessation of large-scale melting of the surrounding ice sheets, the AW could influence the surface waters, independently of the strength of its inflow. Only then was an effective ocean-atmosphere heat transfer enabled, triggering the comparably late onset of the regional Holocene Thermal Maximum. Furthermore, sediment geochemical analyses show that short term cooling events, such as the 8.2 ka event related to the final drainage of glacial Lake Agassiz, lead to glacier advances of the Greenland Ice Sheet. Since the grain size data show that these events had no influence on the AW-inflow to the north eastern Labrador Sea, these advances must have been caused by atmospheric cooling. Consequently, we argue that (i) in this region, surface water-based proxies register AW influence rather than inflow (ii) the AW inflow into the Labrador Sea is controlled by the AMOC, but (iii) its impact on an effective ocean-atmosphere heat transfer was hindered by a prevailing meltwater lens in the early Holocene, i.e. until the cessation of large-scale melting of the surrounding ice sheets

Using novel methods to track British and Irish Ice Sheet dynamics since the Late Pleistocene, along the west Porcupine Bank, NE Atlantic

Extensive research has been undertaken to elucidate the glacial history of the British Irish Ice Sheet (BIIS) in the NE Atlantic. BRITICE-CHRONO has compiled terrestrial and marine based evidence, to provide an empirical reconstruction of ice sheet expansion and retreat during the Late Pleistocene. Across the Irish margin, particular focus has been given to seafloor sediments which contain ice-rafted debris (IRD). However, there are few publications on IRD from areas proximal to the maximum extent of the BIIS, which would offer further insights on the behaviour of the ice sheet during (de)glacial events. Previous exploratory surveys of the west Porcupine Bank (wPB) visually identified IRD on the seafloor and these present a new study site to investigate the extent of the BIIS and the course of its icebergs. Moreover, there are uncertainties about the effects of icebergs on the marine life and cold-water corals occupying the nearby Porcupine Bank Canyon. Assessing a sediment core containing an IRD analogue for the wPB would thus, have a dual purpose. In the past however, coring missions of the wPB using traditional coring methods (i.e. piston and gravity cores) were unsuccessful. Here, we utilized a novel ROV-mounted vibrocoring procedure to capture a 0.75 m IRD-bearing sediment core from the wPB. Then further novel analytical methods (computed tomography-based IRD-detection) were used to quantify IRD every 0.02 cm to provide the highest resolution record of BIIS related IRD to date. From this, several fluxes of IRD deposition onto the wPB between 31.6 and 9 ka BP were revealed and corroborated by other published records from across the NE Atlantic. It was shown that the wPB IRD fluxes occur simultaneously with other parts of the margin. The IRD signal also shows that iceberg calving occurred on the wPB during the Younger Dryas. Grain-size analysis of the core allowed for a reconstruction and interpretation of the palaeoenvironmental conditions during these IRD flux events and shows that BIIS-derived glaciers had a major impact on hydrodynamic conditions in the wPB. Subsequently, intensive scouring led to a major hiatus in the core during 27.3â 17.2 ka BP. These results are a useful addition to BIIS literature on this part of the shelf. Furthermore, it shows that bottom currents were influenced by (de)glacial events, an important finding when considering the presence of nearby current-dependant benthos

Environmental forcing by submarine canyons: Evidence between two closely situated cold-water coral mounds (Porcupine Bank Canyon and Western Porcupine Bank, NE Atlantic)

Within the Porcupine Bank Canyon (NE Atlantic), cold-water coral (CWC) mounds are mostly found clustered along the canyon lip, with individual disconnected mounds occurring nearby on the western Porcupine Bank. Remotely operated vehicle-mounted vibrocoring was utilized to acquire cores from both of these sites. This study is the first to employ this novel method when aiming to precisely sample two closely situated areas. Radiometric ages constrain the records from the early to mid-Holocene (9.1 to 5.6 ka BP). The cores were then subjected to 3D segmented computer tomography to capture mound formation stages. The cores were then further examined using stable isotopes and benthic foraminiferal assemblages, to constrain the paleoenvironmental variation that influenced CWC mound formation of each site. In total, mound aggradation rate in the Porcupine Bank Canyon and western Porcupine Bank was comparable to other Holocene CWC mounds situated off western Ireland. Results derived from multiproxy analysis, show that regional climatic shifts define the environmental conditions that allow positive coral mound formation. In addition, the aggradation rate of coral mounds is higher adjacent to the Porcupine Bank Canyon than on the western Porcupine Bank. Benthic foraminifera assemblages and planktic foraminiferal δ13C reveal that higher quality organic matter is more readily available closer to the canyon lip. As such, we hypothesize that coral mound formation in the region is likely controlled by an interplay between enhanced shelf currents and the existence of the Eastern North Atlantic Water-Mediterranean Outflow Water-Transition Zone. The geomorphology of the canyon promotes upwelling of these water masses that are enriched in particles, including food and sediment supply. The higher availability of these particles support the development and succession of ecological hotspots along the canyon lip and adjacent areas of the seafloor. These observations provide a glimpse into the role that submarine canyons play in influencing macro and micro benthic fauna distributions and highlights the importance of their conservation

Solenosmilia variabilis-bearing cold-water coral mounds off Brazil

Cold-water corals (CWC), dominantly Desmophyllum pertusum (previously Lophelia pertusa), and their mounds have been in the focus of marine research during the last two decades; however, little is known about the mound-forming capacity of other CWC species. Here, we present new 230Th/U age constraints of the relatively rarely studied framework-building CWC Solenosmilia variabilis from a mound structure off the Brazilian margin combined with computed tomography (CT) acquisition. Our results show that S. variabilis can also contribute to mound formation, but reveal coral-free intervals of hemipelagic sediment deposits, which is in contrast to most of the previously studied CWC mound structures. We demonstrate that S. variabilis only occurs in short episodes of < 4 kyr characterized by a coral content of up to 31 vol%. In particular, it is possible to identify distinct clusters of enhanced aggradation rates (AR) between 54 and 80 cm ka−1. The determined AR are close to the maximal growth rates of individual S. variabilis specimens, but are still up to one order of magnitude smaller than the AR of D. pertusum mounds. Periods of enhanced S. variabilis AR predominantly fall into glacial periods and glacial terminations that were characterized by a 60–90 m lower sea level. The formation of nearby D. pertusum mounds is also associated with the last glacial termination. We suggest that the short-term periods of coral growth and mound formation benefited from enhanced organic matter supply, either from the adjacent exposed shelf and coast and/or from enhanced sea-surface productivity. This organic matter became concentrated on a deeper water-mass boundary between South Atlantic Central Water and the Antarctic Intermediate Water and may have been distributed by a stronger hydrodynamic regime. Finally, periods of enhanced coral mound formation can also be linked to advection of nutrient-rich intermediate water masses that in turn might have (directly or indirectly) further facilitated coral growth and mound formation

Environmental forcing by submarine canyons: evidence between two closely situated cold-water coral mounds (Porcupine Bank Canyon and Western Porcupine Bank, NE Atlantic)

Within the Porcupine Bank Canyon (NE Atlantic), cold-water coral (CWC) mounds are mostly found clustered along the canyon lip, with individual disconnected mounds occurring nearby on the western Porcupine Bank. Remotely operated vehicle-mounted vibrocoring was utilized to acquire cores from both of these sites. This study is the first to employ this novel method when aiming to precisely sample two closely situated areas. Radiometric ages constrain the records from the early to mid-Holocene (9.1 to 5.6 ka BP). The cores were then subjected to 3D segmented computer tomography to capture mound formation stages. The cores were then further examined using stable isotopes and benthic foraminiferal assemblages, to constrain the paleoenvironmental variation that influenced CWC mound formation of each site. In total, mound aggradation rate in the Porcupine Bank Canyon and western Porcupine Bank was comparable to other Holocene CWC mounds situated off western Ireland. Results derived from multiproxy analysis, show that regional climatic shifts define the environmental conditions that allow positive coral mound formation. In addition, the aggradation rate of coral mounds is higher adjacent to the Porcupine Bank Canyon than on the western Porcupine Bank. Benthic foraminifera assemblages and planktic foraminiferal d13C reveal that higher quality organic matter is more readily available closer to the canyon lip. As such, we hypothesize that coral mound formation in the region is likely controlled by an interplay between enhanced shelf currents and the existence of the Eastern North Atlantic Water-Mediterranean Outflow Water-Transition Zone. The geomorphology of the canyon promotes upwelling of these water masses that are enriched in particles, including food and sediment supply. The higher availability of these particles support the development and succession of ecological hotspots along the canyon lip and adjacent areas of the seafloor. These observations provide a glimpse into the role that submarine canyons play in influencing macro and micro benthic fauna distributions and highlights the importance of their conservation

Ice sheet–free West Antarctica during peak early Oligocene glaciation

One of Earth’s most fundamental climate shifts – the greenhouse-icehouse transition 34 Ma ago – initiated Antarctic ice-sheet build-up, influencing global climate until today. However, the extent of the ice sheet during the Early Oligocene Glacial Maximum (~33.7–33.2 Ma) that immediately followed this transition, a critical knowledge gap for assessing feedbacks between permanently glaciated areas and early Cenozoic global climate reorganization, is uncertain. Here, we present shallow-marine drilling data constraining earliest Oligocene environmental conditions on West Antarctica’s Pacific margin – a key region for understanding Antarctic ice sheet-evolution. These data indicate a cool-temperate environment, with mild ocean and air temperatures preventing West Antarctic Ice Sheet formation. Climate-ice sheet modeling corroborates a highly asymmetric Antarctic ice sheet, thereby revealing its differential regional response to past and future climatic change

A large-scale transcontinental river system crossed West Antarctica during the Eocene

Extensive ice coverage largely prevents investigations of Antarctica’s unglaciated past. Knowledge about environmental and tectonic development before large-scale glaciation, however, is important for understanding the transition into the modern icehouse world. We report geochronological and sedimentological data from a drill core from the Amundsen Sea shelf, providing insights into tectonic and topographic conditions during the Eocene (~44 to 34 million years ago), shortly before major ice sheet buildup. Our findings reveal the Eocene as a transition period from >40 million years of relative tectonic quiescence toward reactivation of the West Antarctic Rift System, coinciding with incipient volcanism, rise of the Transantarctic Mountains, and renewed sedimentation under temperate climate conditions. The recovered sediments were deposited in a coastal-estuarine swamp environment at the outlet of a >1500-km-long transcontinental river system, draining from the rising Transantarctic Mountains into the Amundsen Sea. Much of West Antarctica hence lied above sea level, but low topographic relief combined with low elevation inhibited widespread ice sheet formation