New insights into endolithic palaeocommunity development in mobile hard substrate using CT imaging of bioeroded clasts from the Pliocene (Almería, SE Spain).

Bioeroded carbonate clasts from a Pliocene shallow-marine succession of Almería (SE Spain, Betic Cordillera) were analysed with computed tomography (CT). This revealed the detailed 3D architecture of bioerosion structures hidden within and allowed for their ichnotaxonomic identification (14 ichnospecies of 5 ichnogenera) and quantification. Borings are produced by worms, mostly polychaetes and sipunculids dominated, followed by bivalves and lastly by sponges. The crosscutting relationship between the borings and their preservation characteristics points to a complex colonization history of the clasts with repeated bioerosive episodes interrupted by physical disturbances, including overturning and abrasion of the clasts followed by their recolonization. Our findings facilitated paleoenvironmental interpretation and can be compared to analogous modern-day ecological succession. The sharp dominance of worm borings - early successional species - may be related to frequent, periodic, physical disturbance that possibly prevented the cobble-dwelling macroboring community from being overtaken by sponges - late successional taxa. CT, hand sample and petrographic observations detected, aside from borings, other irregularly shaped pores which are interpreted to be generated by diagenetic processes including dolomitization, silicification and dissolution, representing an intraparticle moldic and moldic enlarged porosity. Boring porosity crosscutting the diagenetically altered grains suggests the later occurrence of bioerosion processes. Irregular shapes ranging from roughly spherical, elongate sub-polyhedral to amoeboid resemble morphologies produced by modern sponges. Moldic pores possibly acted as primary domiciles for boring sponges, which infested, altered and enlarged pre-existing pores as they grew (as happens in the modern), providing an example of how biological and non-biological processes interacted and together influenced endolithic palaeocommunity development.

Rapid recovery of life at ground zero of the end-Cretaceous mass extinction.

The Cretaceous/Palaeogene mass extinction eradicated 76% of species on Earth1,2. It was caused by the impact of an asteroid3,4 on the Yucatán carbonate platform in the southern Gulf of Mexico 66 million years ago 5 , forming the Chicxulub impact crater6,7. After the mass extinction, the recovery of the global marine ecosystem-measured as primary productivity-was geographically heterogeneous 8 ; export production in the Gulf of Mexico and North Atlantic-western Tethys was slower than in most other regions8-11, taking 300 thousand years (kyr) to return to levels similar to those of the Late Cretaceous period. Delayed recovery of marine productivity closer to the crater implies an impact-related environmental control, such as toxic metal poisoning 12 , on recovery times. If no such geographic pattern exists, the best explanation for the observed heterogeneity is a combination of ecological factors-trophic interactions 13 , species incumbency and competitive exclusion by opportunists 14 -and 'chance'8,15,16. The question of whether the post-impact recovery of marine productivity was delayed closer to the crater has a bearing on the predictability of future patterns of recovery in anthropogenically perturbed ecosystems. If there is a relationship between the distance from the impact and the recovery of marine productivity, we would expect recovery rates to be slowest in the crater itself. Here we present a record of foraminifera, calcareous nannoplankton, trace fossils and elemental abundance data from within the Chicxulub crater, dated to approximately the first 200 kyr of the Palaeocene. We show that life reappeared in the basin just years after the impact and a high-productivity ecosystem was established within 30 kyr, which indicates that proximity to the impact did not delay recovery and that there was therefore no impact-related environmental control on recovery. Ecological processes probably controlled the recovery of productivity after the Cretaceous/Palaeogene mass extinction and are therefore likely to be important for the response of the ocean ecosystem to other rapid extinction events.

Jurassic paleosurfaces with fecal mounds reveal the last supper of arenicolid worms.

Exceptional paleosurfaces preserving fecal casting mounds occur in the Upper Jurassic Lastres Formation of Spain. As in modern shorelines, these biogenic structures are associated with straight to sinuous-crested ripples showing the interplay of biological and physical processes in a low-energy marine environment. These trace fossils display characteristics, distribution, and densities like those of modern arenicolid populations (approximately 35 specimens per m2). Under close examination, these fecal casting mounds are morphologically undistinguishable from those produced by recent arenicolids (e.g. Arenicola marina, Abarenicola pacifica), providing evidence of the presence of these polychaetes in the Late Jurassic. As their modern counterparts, fossil arenicolids very likely modified their environment generating a seabed topography and impacting ancient benthic communities, sediment characteristics, and sediment biogeochemistry. Although the presence of oxic microhabitats and biogeochemical processes cannot be accurately measured in the fossil record, comparison with the work of modern populations allows to make inferences on sediment reworking and bioirrigation potential. In addition, association with grazing trails supports the idea of fertilization and modulation of food resources to other species. These paleosurfaces underscore the significance of high-fidelity snapshots in the fossil record (true substrates) to reconstruct past ecologies and sediment biogeochemistry. A new ichnotaxon, Cumulusichnus asturiensis n. igen. and n. isp., is defined.

Northernmost (Subarctic) and deepest record of Paleodictyon: paleoecological and biological implications.

Paleodictyon is one of the most iconic and widespread of trace fossils in the geological record. However, modern examples are less well known and restricted to deep-sea settings at relatively low latitudes. Here, we report the distribution of Paleodictyon at six abyssal sites near the Aleutian Trench. This study reveals for the first time the presence of Paleodictyon at Subarctic latitudes (51°-53°N) and at depths over 4500 m, although the traces were not observed at stations deeper than 5000 m suggesting that there is some bathymetric constraint for the trace maker. Two small Paleodictyon morphotypes were recognized (average mesh size of 1.81 cm), one having a central hexagonal pattern, the other being characterized by a non-hexagonal pattern. Within the study area, Paleodictyon shows no apparent correlation with local environmental parameters. Finally, based on a worldwide morphological comparison, we conclude that the new Paleodictyon specimens represent distinct ichnospecies that are associated with the relatively eutrophic conditions in this region. Their smaller size may reflect this more eutrophic setting in which sufficient food can be obtained from a smaller area in order to satisfy the energetic requirements of the tracemakers. If so, then Paleodictyon size may provide some assistance when interpreting paleoenvironmental conditions.

Life before impact in the Chicxulub area: unique marine ichnological signatures preserved in crater suevite.

To fully assess the resilience and recovery of life in response to the Cretaceous-Paleogene (K-Pg) boundary mass extinction ~ 66 million years ago, it is paramount to understand biodiversity prior to the Chicxulub impact event. The peak ring of the Chicxulub impact structure offshore the Yucatán Peninsula (México) was recently drilled and extracted a ~ 100 m thick impact-generated, melt-bearing, polymict breccia (crater suevite), which preserved carbonate clasts with common biogenic structures. We pieced this information to reproduce for the first time the macrobenthic tracemaker community and marine paleoenvironment prior to a large impact event at the crater area by combining paleoichnology with micropaleontology. A variable macrobenthic tracemaker community was present prior to the impact (Cenomanian-Maastrichtian), which included soft bodied organisms such as annelids, crustaceans and bivalves, mainly colonizing softgrounds in marine oxygenated, nutrient rich, conditions. Trace fossil assemblage from these upper Cretaceous core lithologies, with dominant Planolites and frequent Chondrites, corresponds well with that in the overlying post-impact Paleogene sediments. This reveals that the K-Pg impact event had no significant effects (i.e., extinction) on the composition of the macroinvertebrate tracemaker community in the Chicxulub region.

The complex case of Macaronichnus trace fossil affecting rock porosity.

Bioturbation is an important factor for reservoir quality due to the modification of host rock petrophysical properties (i.e., porosity, permeability, and connectivity). However, there is no predictable relationship between bioturbation and its effect on rock properties, due to the variability of the involved ichnological features. A detailed ichnological analysis is necessary to determine how bioturbation affects petrophysical properties in a bioturbated reservoir. Traditionally, ichnological features such as density, tiering, size, orientation, architecture, and fill, have been considered. However, other properties have been undervalued as is the case of lining. Here, we present a detailed study on the effects of Macaronichnus burrows, an ichnotaxon usually related to hydrocarbon exploration due to its high concentration in rock notably affecting petrophysical properties. Macaronichnus, a subhorizontal cylindrical burrow, is characterized by a well-defined and developed outer rim surrounding the tube core. Our data indicates a clear zonation in porosity according to burrow structure, with the lowest porosity in the tube core and higher values associated with the surrounded rim. This duality is determined by the tracemaker grain selective feeding activity and the consequent concentrated cementation. The organism concentrates the lighter minerals in the tube core fill during feeding, favoring post-depositional cementation during diagenesis and this results in lower porosity than the host rock. However, heavy minerals, mainly glauconite, are located in the rim, showing higher porosity. Our results support the view that ichnological analyses are essential to determine reservoir quality in bioturbated reservoirs, evidencing that other ichnological properties in addition to those traditionally considered must be evaluated.

The Late Miocene Rifian corridor as a natural laboratory to explore a case of ichnofacies distribution in ancient gateways.

Oceanic gateways have modulated ocean circulation and have influenced climatic variations throughout the Earth´s history. During the late Miocene (7.8-7.35 Ma), the Atlantic Ocean and the Mediterranean Sea were connected through the Rifian Corridor (Morocco). This gateway is one of the few examples of deep ancient seaways with a semi-continuous sedimentary record. Deposits comprise turbidites intercalated between deep-sea mudstone (i.e., hemipelagites and drift deposits), channelized sandstone contourite facies, and shallow marine sandstone. Herein an ichnological analysis was conducted in these upper Miocene sediments to improve characterisation of palaeoenvironmental conditions. In addition, ichnofacies were analysed to elucidate how bottom currents control ichnofacies distribution and can modify their attributes. Turbidite deposits are typified by vertical trace fossils (i.e., Ophiomorpha), conforming the Ophiomorpha rudis ichnosubfacies. Contouritic sandstone exhibits high density and low diversity trace-fossil assemblage, with predominant Macaronichnus and Scolicia, resembling a proximal expression of the Cruziana ichnofacies. Shallow marine environments are dominated by vertical trace fossils (e.g., Conichnus, Ophiomorpha, Skolithos), allowing an assignation to the Skolithos ichnofacies. This study reveals for the first time a variability in ichnofacies attributes and distribution at the Rifian Corridor, associated with turbidites, contourite and shallow marine sediments. Hydrodynamic energy reveals as the major factor controlling trace maker communities in the studied seaway. Highly energetic conditions typical of shallower settings are present in deeper-water environments (i.e., slope), contributing to ichnodiversity impoverishment in ichnofacies.

CroSSED sequence, a new tool for 3D processing in geosciences using the free software 3DSlicer.

The scientific application of 3D imaging has evolved significantly over recent years. These techniques make it possible to study internal features by non-destructive analysis. Despite its potential, the development of 3D imaging in the Geosciences is behind other fields due to the high cost of commercial software and the scarce free alternatives. Most free software was designed for the Health Sciences, and the pre-settled workflows are not suited to geoscientific materials. Thus, an outstanding challenge in the Geosciences is to define workflows using free alternatives for Computed Tomography (CT) data processing, promoting data sharing, reproducibility, and the development of specific extensions. We present CroSSED, a processing sequence for 3D reconstructions of CT data, using 3DSlicer, a popular application in medical imaging. Its usefulness is exemplified in the study of burrows that have low-density contrast with respect to the host sediment. For geoscientists who have access to CT data and wish to reconstruct 3D structures, this method offers a wide range of possibilities and contributes to open-science and applied CT studies.

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.

Trace fossil evidence for infaunal moulting in a Middle Devonian non-trilobite euarthropod.

Trace fossils represent the primary source of information on the evolution of animal behaviour through deep time, and provide exceptional insights into complex life strategies that would be otherwise impossible to infer from the study of body parts alone. Here, we describe unusual trace fossils found in marginal-marine, storm- and river-flood deposits from the Middle Devonian Naranco Formation of Asturias (northern Spain) that constitute the first evidence for infaunal moulting in a non-trilobite euarthropod. The trace fossils are preserved in convex hyporelief, and include two main morphological variants that reflect a behavioural continuum. Morphotype 1 consists of a structure that superficially resembles a Rusophycus with an oval outline that possesses a distinctly three lobed axis with an elevated central ridge and regularly spaced transverse furrows that convey the appearance of discrete body segments. The anterior part is the most irregular region of the structure, and it is not always recorded. Morphotype 2 displays more elongated, tubular morphology. Careful observation, however, reveals that it comprises up to three successive morphotype 1 specimens organised in a linear fashion and partially truncating each other. Trilobate morphology and effaced transverse furrows are locally evident, but the predominant morphological feature is the continuous, elevated ridge. The detailed morphology of morphotype 1 and well-preserved, discrete segments of morphotype 2 closely resemble the dorsal exoskeleton of the enigmatic late Carboniferous euarthropod Camptophyllia, suggesting the possible affinities of the producer. Comparisons with patterns of Devonian phacopid trilobite exuviation suggest that the Naranco Formation trace fossils may have been produced by the infaunal activities of an euarthropod that anchored its dorsal exoskeleton in the firm sediment during the body inversion moult procedure. Our findings expand the phylogenetic and environmental occurrence of infaunal moulting in Palaeozoic euarthropods, and suggest a defensive strategy against predation, previously only known from trilobites preserved in open-marine deposits.

Lateral variability of ichnological content in muddy contourites: Weak bottom currents affecting organisms' behavior.

Although bioturbation is commonly recognized in contourites, only a few studies have analyzed the ichnological content of these deposits in detail. These studies have mainly focused on meso-scale bigradational sequence (a coarsening upward followed by a fining-upward sequence resulting from variations in current velocity). Here we present data from gravitational cores collected along the NW Iberian Margin showing systematic variation in ichnological content across proximal to distal depocenters within a large-scale elongated contourite drift. Data demonstrate that tracemakers' behavior varies depending on the distance relative to the bottom current core. Trace fossils are already known to be a useful tool for studying of contouritic deposits and are even used as criterion for differentiating associated facies (e.g., turbidites, debrites), though not without controversy. We propose a mechanism by which the distance to the bottom current core exerts tangible influence on specific macro-benthic tracemaker communities in contourite deposits. This parameter itself reflects other bottom current features, such as hydrodynamic energy, grain size, nutrient transport, etc. Ichnological analysis can thus resolve cryptic features of contourite drift depositional settings.

High-resolution data from Laser Ablation-ICP-MS and by ICP-OES analyses at the Cretaceous/Paleogene boundary section at Agost (SE Spain).

A high-resolution analysis of the distribution of major and trace elements across the Cretaceous/Paleogene boundary (KPgB) in the distal section of Agost (SE Spain) was performed. The KPgB sediments were drilled to recover a 22 cm-long core; the lower 5 cm corresponding to the uppermost Maastrichtian and the upper 17 cm to the lowermost Danian. The unconsolidated sediments were resin-embedded under O2-free conditions, cut and polished. Laser Ablation-Inductivity Coupled Plasma-Mass Spectrometry (LA-ICP-MS) analyses were conducted at 10 µm increments and a laser-beam of 80 µm. Discrete samples were taken immediately prior to the resin-embedding and analyzed by Inductivity Coupled Plasma-Optical Emission Spectroscopy (ICP-OES). Results obtained by both analytical methods (LA-ICP-MS and ICP-OES) are presented. (Further interpretations and discussion are included in Sosa-Montes de Oca et al., 2018 [6]).