The metabolic core of the prokaryotic community from deep-sea sediments of the southern Gulf of Mexico shows different functional signatures between the continental slope and abyssal plain

Marine sediments harbor an outstanding level of microbial diversity supporting diverse metabolic activities. Sediments in the Gulf of Mexico (GoM) are subjected to anthropic stressors including oil pollution with potential effects on microbial community structure and function that impact biogeochemical cycling. We used metagenomic analyses to provide significant insight into the potential metabolic capacity of the microbial community in Southern GoM deep sediments. We identified genes for hydrocarbon, nitrogen and sulfur metabolism mostly affiliated with Alpha and Betaproteobacteria, Acidobacteria, Chloroflexi and Firmicutes, in relation to the use of alternative carbon and energy sources to thrive under limiting growth conditions, and metabolic strategies to cope with environmental stressors. In addition, results show amino acids metabolism could be associated with sulfur metabolism carried out by Acidobacteria, Chloroflexi and Firmicutes, and may play a crucial role as a central carbon source to favor bacterial growth. We identified the tricarboxylic acid cycle (TCA) and aspartate, glutamate, glyoxylate and leucine degradation pathways, as part of the core carbon metabolism across samples. Further, microbial communities from the continental slope and abyssal plain show differential metabolic capacities to cope with environmental stressors such as oxidative stress and carbon limiting growth conditions, respectively. This research combined taxonomic and functional information of the microbial community from Southern GoM sediments to provide fundamental knowledge that links the prokaryotic structure to its potential function and which can be used as a baseline for future studies to model microbial community responses to environmental perturbations, as well as to develop more accurate mitigation and conservation strategies.

Pleistocene cyclostratigraphy on the continental rise and abyssal plain of the western Ross Sea, Antarctica

<p>This thesis investigates glacimarine sedimentation processes operating on the continental margin of the western Ross Sea during the Pleistocene (˜2.5 Ma). This time period is characterised by a major global cooling step at ˜0.8 Ma, although several proposed episodes of major marine-based Antarctic Ice Sheet (AIS) retreat in warm interglacial periods are inferred to have occurred after this time. Constraining the timing and magnitude of past marine-based AIS retreat events in the Ross Sea through this time will improve our understanding of the forcing mechanisms and thresholds that drive marine-based ice sheet retreat. Identifying such mechanisms and thresholds is crucial for assisting predictive models of potential ice sheet collapse in a future world with rapidly rising atmospheric carbon dioxide (CO₂) concentrations. Six sedimentary cores forming a north-to-south transect from the continental rise to the abyssal plain of the western Ross Sea were examined in order to identify potential sedimentary signatures of past marine-based ice sheet variability and associated oceanographic change. A lithofacies scheme and stratigraphic framework were developed, which allowed the identification of shifting sedimentary processes through time. The sediments are interpreted to have been deposited primarily under the influence of bottom currents, most likely from changing rates of dense Antarctic Bottom Water (AABW) formation over glacial-interglacial cycles. Two dominant lithofacies (laminated and bioturbated) are recognised in the Pleistocene contourite sequences. Laminated facies alongside reduced ice-rafted debris (IRD) fluxes and reduced biological productivity are interpreted to represent expanded ice sheet and sea ice margins during glacial conditions, which acted to restrict surface water ventilation resulting in less oxygenated bottom waters. Conversely, laminated facies alongside reduced IRD fluxes and increased productivity are inferred to represent a reduction of ice shelf and sea ice cover resulting in enhanced AABW formation and sediment delivery. In general, it is interpreted that bioturbated facies in combination with enhanced productivity are common during interglacial conditions, with peaks in IRD associated with ice sheet retreat events leading into interglacial conditions. However, the relationships between laminated and bioturbated facies vary between sites, and facies at most sites generally alternate on timescales exceeding that of individual glacial-interglacial cycles (<100 kyr). Nonetheless, there are clear baseline shifts in the facies distributions through time across the sites, and it is inferred these represent step-like shifts in the ice sheet volume and sea ice processes on the continental shelf and above the study sites during the Pleistocene. This thesis also assesses and compares three independent methodologies of obtaining IRD mass accumulation rates (MARs). The three methodologies include counting clasts >2 mm in x-ray images, the sieved weight percentage of the medium-to-coarse sand fraction (250 µm-2 mm), and volumetric estimates of the > 125 µm sand fraction using a laser particle sizer. The x-ray and sieve methods produced comparable results, while the volumetric estimate, although showing comparable long-term trends, produces a lesser correlation to the other two methods. Spectral analysis of the IRD content and the magnetic susceptibility data series reveals that during the Early Pleistocene (2.5-1.2 Ma) ice discharge into the western Ross Sea was paced by the 41 kyr and 100 kyr cycles of obliquity and eccentricity, respectively. The Mid-Pleistocene Transition (MPT;1.2-0.8 Ma) was characterised by a switch to a higher-frequency, lower-amplitude IRD flux during a long-term period of high power in eccentricity, obliquity and precession (˜23 kyr) observed in the orbital solutions, suggesting a relatively linear response to orbital forcing at this time. The colder climate state of the Late Pleistocene (0.8-0.01 Ma) is characterised by IRD fluctuations modulated primarily by the 100 kyr eccentricity forcing that became dominant by 400 ka. In the western Ross Sea, IRD fluxes show a clear response to the orbital pacing of glacial-interglacial cycles, but are equivocal in identifying the magnitude of ice sheet loss or growth through glacial-interglacial cycles.</p>

Pleistocene cyclostratigraphy on the continental rise and abyssal plain of the western Ross Sea, Antarctica

<p>This thesis investigates glacimarine sedimentation processes operating on the continental margin of the western Ross Sea during the Pleistocene (˜2.5 Ma). This time period is characterised by a major global cooling step at ˜0.8 Ma, although several proposed episodes of major marine-based Antarctic Ice Sheet (AIS) retreat in warm interglacial periods are inferred to have occurred after this time. Constraining the timing and magnitude of past marine-based AIS retreat events in the Ross Sea through this time will improve our understanding of the forcing mechanisms and thresholds that drive marine-based ice sheet retreat. Identifying such mechanisms and thresholds is crucial for assisting predictive models of potential ice sheet collapse in a future world with rapidly rising atmospheric carbon dioxide (CO₂) concentrations. Six sedimentary cores forming a north-to-south transect from the continental rise to the abyssal plain of the western Ross Sea were examined in order to identify potential sedimentary signatures of past marine-based ice sheet variability and associated oceanographic change. A lithofacies scheme and stratigraphic framework were developed, which allowed the identification of shifting sedimentary processes through time. The sediments are interpreted to have been deposited primarily under the influence of bottom currents, most likely from changing rates of dense Antarctic Bottom Water (AABW) formation over glacial-interglacial cycles. Two dominant lithofacies (laminated and bioturbated) are recognised in the Pleistocene contourite sequences. Laminated facies alongside reduced ice-rafted debris (IRD) fluxes and reduced biological productivity are interpreted to represent expanded ice sheet and sea ice margins during glacial conditions, which acted to restrict surface water ventilation resulting in less oxygenated bottom waters. Conversely, laminated facies alongside reduced IRD fluxes and increased productivity are inferred to represent a reduction of ice shelf and sea ice cover resulting in enhanced AABW formation and sediment delivery. In general, it is interpreted that bioturbated facies in combination with enhanced productivity are common during interglacial conditions, with peaks in IRD associated with ice sheet retreat events leading into interglacial conditions. However, the relationships between laminated and bioturbated facies vary between sites, and facies at most sites generally alternate on timescales exceeding that of individual glacial-interglacial cycles (<100 kyr). Nonetheless, there are clear baseline shifts in the facies distributions through time across the sites, and it is inferred these represent step-like shifts in the ice sheet volume and sea ice processes on the continental shelf and above the study sites during the Pleistocene. This thesis also assesses and compares three independent methodologies of obtaining IRD mass accumulation rates (MARs). The three methodologies include counting clasts >2 mm in x-ray images, the sieved weight percentage of the medium-to-coarse sand fraction (250 µm-2 mm), and volumetric estimates of the > 125 µm sand fraction using a laser particle sizer. The x-ray and sieve methods produced comparable results, while the volumetric estimate, although showing comparable long-term trends, produces a lesser correlation to the other two methods. Spectral analysis of the IRD content and the magnetic susceptibility data series reveals that during the Early Pleistocene (2.5-1.2 Ma) ice discharge into the western Ross Sea was paced by the 41 kyr and 100 kyr cycles of obliquity and eccentricity, respectively. The Mid-Pleistocene Transition (MPT;1.2-0.8 Ma) was characterised by a switch to a higher-frequency, lower-amplitude IRD flux during a long-term period of high power in eccentricity, obliquity and precession (˜23 kyr) observed in the orbital solutions, suggesting a relatively linear response to orbital forcing at this time. The colder climate state of the Late Pleistocene (0.8-0.01 Ma) is characterised by IRD fluctuations modulated primarily by the 100 kyr eccentricity forcing that became dominant by 400 ka. In the western Ross Sea, IRD fluxes show a clear response to the orbital pacing of glacial-interglacial cycles, but are equivocal in identifying the magnitude of ice sheet loss or growth through glacial-interglacial cycles.</p>

Oceanic strike-slip faults represent active fluid conduits in the abyssal sub-seafloor

We present pore-fluid geochemistry and heat-flow data along the SWIM1 fault in the Horseshoe Abyssal Plain (northeastern Atlantic Ocean). The SWIM1 fault is part of the transcurrent plate boundary between Africa and Eurasia and cuts through as much as 5-km-thick sediments overlying &gt;140 Ma oceanic lithosphere. In a number of places, restraining segments (as long as 15 km) of the SWIM1 fault generate anticlines (positive flower structures) that protrude as ~100-m-high hills above the abyssal plain. Heat flow and gradients of dissolved constituents in pore water are enhanced at these seafloor highs. Transport-reaction modeling confirms that slow advection of deep-seated fluids, depleted in Mg and enriched in Sr and CH4, can explain the observations. The geochemical signature is similar to the one observed at deep-sea mud volcanoes located eastward on the SWIM1 fault. The upward-migrating fluids have interacted with carbonate rocks at maximum 5 km depth, which represent the oldest sedimentary unit on top of the basement. We argue that deep-rooted fluids can generally be mobilized and transported upward along flower structures that formed in restraining-bend segments of long strike-slip faults. Such tectonic settings represent largely unrecognized corridors for mass exchange between lithosphere and ocean.

Geostatistical Seismic Inversion for Temperature and Salinity in the Madeira Abyssal Plain

A two-dimensional multichannel seismic reflection profile acquired in the Madeira Abyssal Plain during June 2016 was used in a modeling workflow comprising seismic oceanography processing, geostatistical inversion and Bayesian classification to predict the probability of occurrence of distinct water masses. The seismic section was processed to image in detail the fine scale structure of the water column using seismic oceanography. The processing sequence was developed to preserve, as much as possible, the relative seismic amplitudes of the data and enhance the shallow structure of the water column by effectively suppressing the direct arrival. The migrated seismic oceanography section shows an eddy at the expected Mediterranean Outflow Water depths, steeply dipping reflectors, which indicate the possible presence of frontal activity or secondary dipping eddy structures, and strong horizontal reflections between intermediate water masses suggestive of double diffuse processes. We then developed and applied an iterative geostatistical seismic oceanography inversion methodology to predict the spatial distribution of temperature and salinity. Due to the lack of contemporaneous direct measurements of temperature and salinity we used a global ocean model as spatial constraint during the inversion and nearby contemporaneous ARGO data to infer the expected statistical properties of both model parameters. After the inversion, Bayesian classification was applied to all temperature and salinity models inverted during the last iteration to predict the spatial distribution of three distinct water masses. A preliminary interpretation of these probabilistic models agrees with the expected ocean dynamics of the region.

Biogeography and Connectivity Across Habitat Types and Geographical Scales in Pacific Abyssal Scavenging Amphipods

Recently, there has been a resurgent interest in the exploration of deep-sea mineral deposits, particularly polymetallic nodules in the Clarion-Clipperton Zone (CCZ), central Pacific. Accurate environmental impact assessment is critical to the effective management of a new industry and depends on a sound understanding of species taxonomy, biogeography, and connectivity across a range of scales. Connectivity is a particularly important parameter in determining ecosystem resilience, as it helps to define the ability of a system to recover post-impact. Scavenging amphipods in the superfamilies Alicelloidea Lowry and De Broyer, 2008 and Lysianassoidea Dana, 1849 contribute to a unique and abundant scavenging community in abyssal ecosystems. They are relatively easy to sample and in recent years have become the target of several molecular and taxonomic studies, but are poorly studied in the CCZ. Here, a molecular approach is used to identify and delimit species, and to investigate evolutionary relationships of scavenging amphipods from both abyssal plain and deep (&gt;3000 m) seamount habitats in three APEIs (Areas of Particular Environmental Interest, i.e., designated conservation areas) in the western CCZ. A total of 17 different morphospecies of scavenging amphipods were identified, which include at least 30 genetic species delimited by a fragment of the cytochrome c oxidase subunit I (COI) barcode gene. The scavenging communities sampled in the western CCZ included the most common species (Abyssorchomene gerulicorbis (Shulenberger and Barnard, 1976), A. chevreuxi (Stebbing, 1906), Paralicella caperesca Shulenberger and Barnard, 1976, and P. tenuipes Chevreux, 1908) reported for other ocean basins. Only four morphospecies, representing five genetic species, were shared between APEIs 1, 4, and 7. The two abyssal plain sites at APEIs 4 and 7 were dominated by two and three of the most common scavenging species, respectively, while the APEI 1 seamount site was dominated by two species potentially new to science that appeared to be endemic to the site. The presence of common species in all sites and high genetic diversity, yet little geographic structuring, indicate connectivity over evolutionary time scales between the areas, which span about 1500 km. Similar to recent studies, the differences in amphipod assemblages found between the seamount and abyssal sites suggest that ecological conditions on seamounts generate distinct community compositions.