What Is The Continental Rise

Continental Rise

The continental ascension on a passive continental margin is a zone of sediment deposition on slopes that are typically between 1:50 and 1:500 and occurs beyond the steeper continental slope, which is usually incised by canyons.

From: Encyclopedia of Geology , 2005

SEDIMENTARY PROCESSES | Deep Water Processes and Deposits

D.J.W. Piper , in Encyclopedia of Geology, 2005

The Deep Continental Margin

The continental rise on a passive continental margin is a zone of sediment deposition on slopes that are typically between 1 : l and 1 : 500 and occurs beyond the steeper continental slope, which is commonly incised past canyons. The continental rise consists principally of submarine fans. An erosional submarine canyon leads to a submarine fan valley or aqueduct, generally with depositional levees and a downslope subtract in aqueduct depth. Depositional lobes are developed at the downslope termination of the aqueduct and pass gradually into the apartment abyssal obviously, where the slope of less than 1 : k would be imperceptible to an observer on the ground. In more complex convergent continental margins in that location is generally more morphological command of deposition, but the same architectural elements are present: erosional canyons lead to leveed channels, channel-termination lobes, and, finally, basin or trench floors with more sail-like stratigraphy. Channels may link ii morphologically distinct basins, equally in the example of the Biscay and Iberia abyssal plains. Indeed, these elements are also present on circuitous continental slopes with intraslope basins.

The channel systems are pathways for turbidity currents. Many channels are highly meandering; whether a channel is meandering or directly appears to exist related to the gradient. On many passive-margin submarine fans, such as the Amazon, Zaire, and Bengal fans, channels modify course abruptly through avulsion. Where channels are topographically constrained, for instance Bounty Channel east of New Zealand, Surveyor Channel in the north-east Pacific, and the Northward-due west Atlantic Mid-Bounding main Channel in the Labrador Sea, an near constant path has been maintained over thousands of kilometres. The lower parts of the turbidity currents transport sand through the channels and, owing to flow expansion, deposit it rapidly as aqueduct-termination lobes. But fine-grained sediment is transported to the distal parts of basins. The upper parts of the current entrain water, thicken, and spill over the levees, depositing overbank silts and muds. The cross-sectional areas of channels (kilometres wide, tens to hundreds of metres deep) and the velocities of turbidity currents estimated from cable breaks (2–19 m s^−one) bear witness to the size and power of the turbidity currents.

Sediment drifts are large sediment bodies constructed by deposition, generally of hemipelagic sediment, from the deep-water thermohaline circulation. The well-nigh powerful currents, in constrictions or adjacent to steep slopes, volition commonly winnow or erode sediment, some of which may exist of turbidite origin. This sediment, together with material introduced into nepheloid layers from the tails of turbidity currents or from resuspension on the shelf, is deposited slowly in areas of lesser electric current velocity. The North Atlantic Ocean, the Mediterranean Sea outlet, the eastern sides of the North and S American continents, the New Zealand margin, and the west side of the Antarctic Peninsula have peculiarly well-adult sediment drifts. Almost have accumulated over many millions of years and take undergone alternating phases of deposition and winnowing or erosion equally bottom-water circulation has fluctuated with irresolute climate. Many sediment drifts support mesoscale bedforms, including erosional furrows and sediment waves, with wavelengths of kilometres and heights of tens of metres.

Read full chapter

URL:

https://www.sciencedirect.com/science/article/pii/B0123693969004962

Submarine Geomorphology☆

N.C. Mitchell , in Reference Module in Earth Systems and Environmental Sciences, 2015

The Continental Ascent and Sedimentary Fans

The continental rises are low-relief aprons to the continental slopes, extending to, and merging indistinctly with, the abyssal plains ( Effigy 1(a) ). They comprise debris shed from the continents past sedimentary flows and fallout of particles from suspension (hemipelagites). Off the Us East Coast, in the uppermost ascension, pits upwardly to 75 m deep occur below gradient gullies and have been interpreted every bit produced by impacts of droppings flows or hydraulic jumps of sedimentary flows emerging from the gullies (Lee et al., 2002). The rise has a more than subdued relief than the continental gradient but with irregularities due to deposits of debris flows (Pratson and Laine, 1989; debrites). In other continental margins, the rise may contain different sediment types only in general usually has subdued relief away from the sedimentary fans. Sedimentary bedforms created by deep-ocean currents commonly occur, as described under 'Abyssal Plains.'

Sedimentary fans occur below the continental slopes, where sedimentary flows leaving gradient canyons become less constrained laterally. Fans class some of the largest sedimentary features on World, for example, the Bengal Fan extends for 3000 km. Channels on fans can appear similar to river channels, every bit they are likewise sinuous and flanked by levees (Damuth et al., 1988). Withal, more detailed analysis has revealed important differences. For example, seismic reflection data suggest that such channels do not migrate laterally (swing) with fourth dimension as practise rivers, perhaps a outcome of internal pressure level gradients setting up different helical movements in passing flows (Peakall et al., 2000). The channel floors can prevarication above the elevation of the surrounding fan. Where the levees have failed, allowing a new channel to form in the levee break, the new channel can have a steep slope, producing a knickpoint that can migrate upstream with erosion like a river channel knickpoint (Pirmez et al., 2000). Given the importance of fans equally oil and gas reservoirs, the industry has nerveless enormous amounts of iii-D seismic data in these areas, which are extremely useful for research on channels (Posamentier and Kolla, 2003).

Read full chapter

URL:

https://www.sciencedirect.com/science/article/pii/B9780124095489092496

Cenozoic history of Antarctic glaciation and climate from onshore and offshore studies

Robert G. McKay , ... Trevor Williams , in Antarctic Climate Development (Second Edition), 2022

3.4.two Amundsen Sea

The Amundsen Sea continental shelf and rise (Fig. iii.9) developed after the Cretaceous pause-up of Zealandia from West Antarctica (see Section three.ii), but the paleoenvironment from the Cretaceous to the Neogene in the Amundsen Sea sector remains poorly sampled. The showtime drill cores from the shelf were collected during a MeBo seabed drilling trek in 2017 (Gohl et al., 2017). A drill core from the inner/heart shelf showed evidence for a temperate rainforest and swamp surround formed in a rift basin in the middle Cretaceous, with a ~xl-Myr hiatus separating these sediments from an overlying sandstone formation of belatedly Eocene historic period (Klages et al., 2020). More analyses of core samples from the other MeBo drill sites, which according to preliminary historic period estimates bridge diverse time slices from the Oligocene to Holocene (Gohl et al., 2017), are in progress. In terms of paleoclimate-related and glacially-driven sedimentary processes in the Neogene and Fourth, this region is dominated by sediment erosion, transport and deposition driven mainly by the large outlet water ice streams of the Pine Isle, Thwaites, Haynes, Pope and Smith glaciers of the eastern Amundsen Sea Embayment. Too as these, many smaller glaciers drain water ice from the elevated central Marie Byrd Land and feed into the Dotson Ice Shelf and the various segments of the Getz Ice Shelf of the central and western Amundsen Sea.

The analysis of seismic lines crossing the slope and shelf, most of them collected since 2000 (Dowdeswell et al., 2006; Graham et al., 2009; Hochmuth and Gohl, 2013; Klages et al., 2014, 2015; Lowe and Anderson, 2002; Uenzelmann-Neben et al., 2007; Weigelt et al., 2012, 2009), resulted in a seismic stratigraphic model (Gohl et al., 2013) very much coordinating to the dated Ross Bounding main shelf stratigraphy with a similar seismic reflection signature (Fig. iii.9). Still, deep drill sites currently do non exist on the Amundsen Body of water shelf and this was a key focus of IODP Trek 379, only a near 'worst-instance scenario' ocean ice season during the drilling window in 2018/19 precluded admission of the drill ship to the continental shelf (Gohl et al., 2021). However, the proposal targets remain viable and important drilling targets for the time to come and will assist in validating these correlations to the Ross Body of water (Fig. three.8). This will determine whether the ice sheet histories in these regions are fundamentally different, with local influences by tectonic processes potentially playing as an of import function equally climate and oceanographic processes. Existing seismic coverage indicates the total pre-glacial to glacial sediment cover on the shelf is upwards to vii km thick in places. Based on the observation of glacially-driven truncational unconformities (surfaces Ass-u4 and above; Fig. 3.nine), an early advance of grounded ice onto the continental inner to middle shelf is interpreted to have not occurred prior to the Miocene. A big proportion of the nowadays outer shelf consists of a 70 km broad zone of prograding sequences that were deposited subsequently transport by advancing grounded ice (Gohl et al., 2013; Hochmuth and Gohl, 2019, 2013).

Studies of the big number of samples from conventional coring systems, and geomorphological studies, take yielded a reasonably detailed record of the ice retreat in the Amundsen Sea since the LGM. These works were comprehensively synthesised by Larter et al. (2014) every bit function of the RAISED consortium project, although numerous new studies accept provided new insights (Klages et al., 2017, 2014; Kuhn et al., 2017; Smith et al., 2017). Combined, these works indicate that the WAIS retreated speedily from the outer shelf at the LGM (almost 18 ka) to the inner shelf at well-nigh ten ka, where it halted until the current retreat from coastal locations started in the mid-20th century (Hillenbrand et al., 2013; Klages et al., 2017; Larter et al., 2014; Smith et al., 2017, 2014). Sedimentary, geochemical and microfossil proxies from mail-LGM sediment cores on the continental shelf suggest incursions of relatively warm Circumpolar Deep Water onto the shelf forced deglaciation during the early on Holocene, besides as water ice shelf thinning since the mid-20th century (Hillenbrand et al., 2017; Minzoni et al., 2017). Circumpolar Deep Water incursions accept been identified as the main driver for present ice shelf cavity melting in the Amundsen Sea embayment (e.one thousand., Nakayama et al., 2013; Scambos et al., 2017). Nowadays research and future drilling in this region are consequently focusing on testing the hypothesis that Circumpolar Deep Water incursions onto the shelf were the main commuter of WAIS retreat during past warm periods of the Fourth and Neogene (Gohl et al., 2021).

Similar to the continental shelf, the coverage of seismic lines on the continental rise and deep sea of the Amundsen Bounding main has increased in recent years ( Fig. iii.ix), although large unsurveyed areas still exist, in particular in the central and western Amundsen Sea. A single transect along the rising from the Ross Sea to the Amundsen Sea was used to establish the showtime seismic stratigraphic tape on the total sedimentary cover for the western Amundsen Sea with the identification of singled-out pre-glacial, transitional and full glacial sequences (Fig. 3.x) (Lindeque et al., 2016). The seismic stratigraphy was derived by long-distance correlation to the Ross Bounding main chronostratigraphic record based on the shelf drill sites of DSDP Leg 28 (e.g., De Santis et al., 1999; Hayes et al., 1975). The transitional period includes the tardily Eocene to mid-Miocene when grounded water ice first expanded onto the continental shelves, while the full glacial period describes the interval from the mid-Miocene to Quaternary, with intensified ice advances onto the outer shelves. No apparent difference in the deep-sea sedimentation transport processes or temporal shift in degradation between the Amundsen Ocean and Ross Bounding main is observed. Boosted new seismic data were nerveless on a Russian expedition in 2019 and volition help quantify the extent of the depositional sequences in the western Amundsen Sea.

The continental ascension of the eastern Amundsen Sea is dominated past big contourite drifts, some of them ascension several hundred metres above the surrounding seafloor (Nitsche et al., 2000; Scheuer et al., 2006a,b; Uenzelmann-Neben, 2018; Uenzelmann-Neben and Gohl, 2014, 2012; Yamaguchi et al., 1988) (Fig. 3.10). Nigh drift systems are elongated in a north–due south management and flanked by deep-body of water channels eroded by turbidity currents carrying suspended detritus supplied past downslope transport processes from the slope and shelf. The suspended particles were entrained in strong bottom currents and were after deposited on the flanks of the channels to form the drifts. Seismic analyses with first estimates of a chrono-stratigraphy by long-altitude and jump correlation of seismic horizons to DSDP and ODP drill sites in the Bellingshausen Sea and Ross Sea indicate that early drift formation by enhanced lesser-current activity began in the Eocene/Oligocene. Drift formation intensified in the Miocene, likely acquired by expansion of the WAIS during global cooling, increased bounding main-ice comprehend and, as a effect, the formation of Antarctic Bottom Water and enhanced bottom-electric current flow (Uenzelmann-Neben and Gohl, 2012).

Although only two sites on the continental rise and none on the shelf could exist drilled during IODP Trek 379 in 2019 (Fig. iii.i), well-nigh continuous sequences spanning the latest Miocene to Pleistocene, deposited with high sedimentation rates, were recovered from the lower and upper western flank of the Resolution Drift (Gohl et al., 2021). The cores from both sites recovered predominantly glaciomarine, fine-grained terrigenous sediments intercalated with pelagic and hemipelagic deposits that, in the younger parts of the cores, contain more than biogenic material. The records show an interplay of glacially transported shelf sediments that were transported downslope to the continental rise past gravitational processes and redistributed across the rise past turbidity and bottom currents. Although IODP Expedition 379 was unable to call up cores from the shelf, the drill records from the continental rise reveal the timing of glacial advances across the shelf and, thus, the being of a large water ice sheet in West Antarctica for prolonged time periods since the tardily Miocene. Detailed analyses of the IODP Expedition 379 core samples and data are in progress (Gohl et al., 2021).

In dissimilarity to the Amundsen Sea continental shelf, information from conventional sediment cores from the continental gradient and ascent is sparse. Dowdeswell et al. (2006) reported droppings and grain flow deposits presumably of LGM age from 3 curt cores collected on the continental slope. In a long gravity core from the continental rise (Hillenbrand et al., 2002), thick beds of turbidites and contourites deposited during tardily Pleistocene glacial periods alternate with sparse beds of foraminifera-bearing, bioturbated muds deposited during interglacials. A condensed, just well dated core from a seamount location on the continental slope retrieved sediments mainly consisting of pelagic cloth, i.e. planktic foraminifera and iceberg-rafted debris. Notably, this record did non provide evidence for a WAIS collapse during the terminal 800 ka (Hillenbrand et al., 2002). By comparing palaeoceanographic information from a core recovered from the same seamount location to other Antarctic ocean cores, Williams et al. (2019) institute bear witness that deep- and bottom-water mass germination varied betwixt the different sectors of the Antarctic margin during glacial periods of the last 800 ka. Hillenbrand et al. (2009) analysed Pleistocene to Holocene glacial-interglacial cycles in a sediment core recovered from the Resolution Drift (site located to the south of the ii IODP Trek edition 379 drill core sites) and found, based on proxies for biological productivity and lithogenic sediment supply, that in the Amundsen Bounding main the interval from Marine Isotope Stage (MIS) 15 to MIS xiii (621–478 ka) was a single prolonged interglacial catamenia, during which the WAIS may take collapsed. A palaeoceanographic study on the same core by Konfirst et al. (2012) concluded that after the stop of the Mid-Pleistocene Transition (MPT) at ca. 620 ka the Amundsen Body of water low pressure arrangement shifted farther southward during interglacial periods than during before interglacials, thereby increasing Circumpolar Deep Water upwelling onto the shelf. Recent provenance studies provide information about the sub-ice geology and drainage systems in the hinterland that may allow past ice sheet retreat events to be identified in offshore sediments (Simões Pereira et al., 2018, 2020).

Read full affiliate

URL:

https://www.sciencedirect.com/science/article/pii/B9780128191095000086

Methane Hydrate and Submarine Slides

J. Mienert , in Encyclopedia of Sea Sciences (Second Edition), 2009

Seafloor bathymetry and reflection seismic data from continental slopes and rises of the Atlantic, Pacific, Indian, and Black Sea reveal major slide scars and slumps overlying gas hydrate deposits. Slides and slumps shape the morphology of world ocean margins and represent a major machinery for transferring enormous amounts of sediment material over a relatively short fourth dimension, between hours and years, into the deep body of water. The trigger mechanisms of the slides and slumps are oft unknown and atomic number 82 to speculations near various mechanisms. Massive and rapid releases of gases and water from methane hydrate decompositions can be one of the triggers leading to gradient instability along continental-margins. Submarine slides and slumps along continental margins are potential areas for significant releases of methane entering the atmosphere. Our nowadays observations and hypotheses from continental-margin methyl hydride hydrates and slumps are insufficient to quantify any of these major processes on a regional or global scale. Developments in thermodynamic modeling of the temperature processes coupled with analysis of pore-pressure level generation and dissipation in areas with signs of slumps and methane hydrate allow for assessments of today's state of affairs. New theories for combined or coupled processes of bounding main methane hydrate, slides, and climate may come from a less-obvious field, the methane ice core records.

Read total chapter

URL:

https://www.sciencedirect.com/science/article/pii/B9780123744739007049

Seafloor Geomorphology—Coast, Shelf, and Completeness

Peter T. Harris , in Seafloor Geomorphology as Benthic Habitat, 2012

Abyssal Plain

Sediment deposited adjacent to the continents forms the continental ascension. Seaward of this, the state-derived sediment wedge becomes thinner and the morphology gives way to the flat deep-sea plains that are underlain by basaltic bounding main crust ( Figure 6.xi). Abyssal plains are remarkably flat, having a slope of less than 1:one,000 (or <1 m change in height over a distance of one km), considering of the thick sediment drape that covers and subdues most of the underlying basement topography. Ocean basins that receive the greatest sediment input take the best developed abyssal plains (e.g., the Atlantic and Indian oceans and the Gulf of Mexico). Abyssal plains are less well adult in the North Pacific and Southern ocean basins considering these areas do not receive every bit much land-derived sediment. This, in turn, is because of the lack of big river systems draining into them, and/or because of large ocean trench systems that intercept and trap land-derived sediments.

Hilly deep-sea plains are much more common and cover over 100 one thousand thousand km² of the seafloor (Effigy vi.11; Tabular array 6.2), comprising the Earth'due south largest geomorphic characteristic type by surface area. The hilly morphology results from the expression of underlying subsided but hilly basaltic ocean chaff, which is evident through the thin sediment cover.

Read total chapter

URL:

https://www.sciencedirect.com/science/article/pii/B9780123851406000062

Seafloor geomorphology—coast, shelf, and abyss

Peter T. Harris , in Seafloor Geomorphology as Benthic Habitat (Second Edition), 2020

Deep-sea patently and manganese nodules

Sediment deposited adjacent to the continents forms the continental ascension, covering around 10% of the ocean floor. Seawards of the rise the land-derived sediment wedge becomes thinner, and the morphology gives fashion to the apartment abyssal plains that are underlain by basaltic body of water crust ( Fig. half dozen.11). Abyssal plains are remarkably apartment, having a slope of less than 1:g (or less than i yard change in height over a altitude of 1 km), because of the thick sediment drape which covers and subdues most of the underlying basement topography. Ocean basins that receive the greatest sediment input take the all-time developed abyssal plains (e.g., the Atlantic and Indian Oceans and the Gulf of United mexican states). Abyssal plains are less well developed in the Pacific Bounding main basins because land-derived sediment shed past adjacent continents is captured in sea trenches and troughs, from which it is subducted into the pall.

At abyssal depths manganese nodules are formed on the seabed past the straight precipitation of metal hydroxides from seawater. They grow incredibly slowly, at rates of millimeters per million years and they remain unburied by the pelting of hemipelagic sediment from higher up because they occur only at slap-up depth, below the carbonate compensation depth. They generally contain manganese, nickel, copper, and cobalt, although the relative concentrations of the different elements varies from place to place in the sea (SPC, 2013). Mining the nodules has been discussed for many decades because of the valuable metals they incorporate, but mining has non happened then far, mainly because there are other sources of these metals on land where production costs are much cheaper. The instance study presented in Chapter 58, Manganese nodule fields from the NE Pacific as benthic habitats, discusses the benthic habitat provided past manganese nodules located in the Blaring–Clipperton Zone of the northeast Pacific.

Read full affiliate

URL:

https://www.sciencedirect.com/science/article/pii/B9780128149607000063

Mod Glaciomarine Environments and Sediments

E.W. Domack , R. Powell , in By Glacial Environments (Second Edition), 2018

seven.12.2 IODP Expedition 318 and the remarkable Site 1361

Site 1361 was in 3466 m of h2o on the continental rise and targeted ane of the larger of the sediment drifts found forth the Wilkes Land Margin of East Antarctica ( Fig. seven.xi). This site targeted a climate history for the region that was expected to recover from the heart Miocene to the Pleistocene and evaluate the stability of the East Antarctic Ice Sheet during presumed warm periods (e.g., Miocene Climate Optimum, early on Pliocene and Pleistocene MIS stages 31 and 11.

A remarkable succession of lithofacies was recovered at Site 1361 (Fig. 7.53). The succession was analysed using very sophisticated methods including a palaeo magnetic scale from the core that was and so calibrated to the Astronomical fourth dimension scale for palaeomagnetic events in the Late Cenozoic (Fig. seven.53), precise counts of IRD in the 250 to 2000 µm size interval, as converted to flux values, and the ratio of the elements Ba to Al to marker oscillations from biogenic to terrigenous sedimentation, respectively. All these parameters where then compared to a detailed lithofacies log – a log that demonstrated upward increases in diatomite or diatomaceous muds and upward decreases in mud or dirt intervals. These two facies were broken down into cycles and, forth with the IRD bend, provided a key reference department to lay against calculated parameters of summer energy in various time windows (derived from obliquity and precession cycles). The eccentricity cycles of 100 and 200 ka years were as well included (Fig. 7.53). The climate evolution of the EAI region was then institute to contain several of the global elements that marking changes in the Neogene and into the Quaternary, such as the well-known transition from 40,000-year cycles to 100,000-year cycles marked within a transitional interval virtually three.4 to 3.0 million years ago (Patterson et al., 2014). That GM sediment from this region signal (from the IRD solitary) that the EAIS participated in this change of climate periodicity is a great step forward (Fig. 7.54).

Read full chapter

URL:

https://www.sciencedirect.com/science/article/pii/B9780081005248000300

Tropical and Sub-Tropical West Africa

P. Giresse , in Developments in Quaternary Sciences, 2008

Site of Ivory-Coast

The site is located on the lower part of the continental ascent (3,500 m water depth). The eight-m long gravity core covers eighteen,000–225,000 year; the uppermost samples record the transition betwixt isotope Stages 2 and 1. Unfortunately, there are no data regarding pollen influx rates. The tropical rain forest and swamp wood pollen are well represented in the deposits and may be current of air likewise every bit river-borne (Frédoux, 1994). Maximal extent of open mural (dry forest and savanna) was recorded for Stages vi and 2, and the ends of Stage 4 (Fig. 2). Fifty-fifty during these critical periods the rain forest decreased but did not disappear completely; diverse biotopes were preserved in a series of refugia.

Pelting woods elements only autumn below 20% of the total pollen assemblage during Phase six. Maximal extension of the lowland rainforest is recorded during substages 5.v, five.i, and in the earliest Holocene. Rhizophora pollen percentages show peaks over fifty% during almost the same isotopic Stages 7, 5.5, and Termination I, whereas very low percentages (less than 10%) are plant for Stages 4 and two. Maxima in Podocarpus pollen percentages are recorded for substages five.4, 5.ii and 5.i and are regarded as a noteworthy marking of "cloud wood" extent during boundary phases marked by relative cooling. These phases developed during interglacial periods when cooler and wetter conditions were prevalent on the higher relief of the Guinean Ridge.

Some other core in the same region records the response of tropical atmospheric circulation to global palaeoclimatic events over the Last Glacial/Interglacial transition (Lézine et al., 1994). Pollen assemblages demonstrate the intensity of the southern atmospheric circulation over equatorial West Africa during the dry period with Artemisia and Ephedra of Saharan origin, showing maximum values effectually 15,000 and x,300 yr BP suggesting maximum wind-diddled action. In contrast, the early Holocene humid phase, ca. 8,500 yr BP, is characterised by the absence of the Saharan taxa suggesting both a weakening in the continent ocean aeolian send and growing monsoon fluxes. During the offset phases of this transition, Podocarpus percentages are very low and and then reached their highest values ca. viii,500 yr BP.

Read total chapter

URL:

https://www.sciencedirect.com/science/article/pii/S1571086608800140

Deep-Sea Ecology

Tracey T. Sutton , Rosanna J. Milligan , in Encyclopedia of Ecology (Second Edition), 2019

The Continental Slopes (Bathyal Zone)

The continental slopes extend gradually from the continental shelf suspension to the continental rise (c. 3000 m depth), with an boilerplate gradient of around four°. Covering only 15% of the Earth׳s surface, the continental slopes are mostly covered past soft sediments, simply are punctuated by a diverse range of geological features, such every bit seamounts, canyons, hydrothermal vents, common cold seeps, and alkali pools. Where the physical and chemical conditions are suitable, cold-water coral reefs, sponge beds or other emergent epifauna (organisms living on the sediment surface) may generate large, circuitous frameworks that provide a greater diverseness of habitats for a range of invertebrate and vertebrate taxa. Amid the deep-ocean benthic habitats, continental slopes are where the greatest variability in oceanographic conditions occurs, which can strongly affect the distribution of the benthic fauna. For example, oxygen minimum zones can lead to a reduced biodiversity of benthic animals, but an increased abundance of hypoxia-tolerant species. Similarly, varying current speeds may affect the coarseness of the benthic sediments or the deposition of "marine snow" (agglutinated flocs of dead plankton tissue and biological waste material) to the seafloor, all of which may impact the abundance and diversity of the creature living on and in the seabed.

At the upper (shallowest) end of the continental slopes, the animal generally resembles that found in coastal environments, but equally one goes deeper we see progressive shifts in morphology and life-history traits with increasing depth. Demersal fishes, for example, testify a general shift towards elongate, eel-like body shapes with increasing depth, which are better suited for conserving free energy in a nutrient-poor surroundings. The systematic grade of fishes likewise changes, with the more "avant-garde" spiny-rayed taxa (east.m., seabasses, scorpionfishes) giving mode to more "basal" taxa (e.g., eels, cusk eels, and cod-similar fishes). This trend is mirrored past the cartilaginous fishes, with requiem sharks (e.chiliad., bull, tiger, and lemon sharks) and rays being replaced by basal taxa similar catsharks and chimaeras.

Read full chapter

URL:

https://world wide web.sciencedirect.com/science/commodity/pii/B9780124095489110103

Globe and Life Processes Discovered from Subseafloor Environments

Carlota Escutia , ... the Expedition 318 Scientists, in Developments in Marine Geology, 2014

3.3.2.4 The Icehouse: Oligocene to Pleistocene Records of EAIS Variability

Icehouse records of EAIS variability and related paleoceanographic changes were recovered from continental ascent Sites U1356, U1359, and U1361, and continental shelf Site U1358 ( Figure 3.3.3(A) and (B)). Drilling at continental rise Site U1356 recovered a thick department of Oligocene to upper Miocene sediments (Figures 3.3.3, 3.3.5, and iii.3.8) indicative of relatively deep water, sea ice—influenced setting (Escutia et al., 2011; Houben et al., 2013). In general, sediments at this site record a long-term cooling trend from the early Eocene (54–52 Ma) to the middle Miocene (15-13 Ma) on the order of 8 °C (Passchier et al., 2013). Within this trend, Oligocene to upper Miocene sediments are indicative of episodically reduced oxygen conditions either at the seafloor or inside the upper sediments prior to ∼17 Ma. I of the major climate transitions during this menses is the Oligocene–Miocene transition leading to the Mi-1 event (Figure 3.3.1). This climate transition coincides with one of the major regional unconformities in the Wilkes Land margin, unconformity WL-U5 (Escutia, Eittreim, & Cooper, 1997,. 2005). The cooling trend leading to the Mi-i event glaciation is characterized past mass transport processes (Salabarnada, Escutia, Nelson, Damuth, & Brinkhuis, 2014). Following the Mi-1 consequence, sedimentation is characterized past hemipelagic, turbidity-, and bottom-electric current deposition (Escutia, Donda, Lobo, & Tanahashi, 2007; Escutia et al., 2002; 2011; Salabarnada et al., 2014). From the tardily early Miocene (∼17 Ma) onward, progressive deepening and possible intensification of deep-water menstruum and circulation lead to a transition from a poorly oxygenated depression-silica system (present from the early to early on eye Eocene to late early on Miocene) to a well-ventilated silica-enriched system akin to the mod Antarctic ocean (Escutia et al., 2011).

A complete record with good recovery (80–100%) of late Miocene to Pleistocene deposits was obtained from continental ascension Sites U1359 and U1361 (Figures 3.three.3, three.3.viii(A) and (B)). These Sites were drilled on the top of the east levee of the Jussieau Aqueduct on a proximal (3009 mbsl) and distal (3454 mbsl) position, respectively (Escutia et al., 2011). The records from both Sites should be therefore complimentary. Shipboard, relatively loftier amplitude variations in sedimentological, wireline logging, and magnetic susceptibility data indicated a stiff potential for this record to reveal EAIS dynamics down to orbital timescales (105 and 41 yard.y. cyclicity) (Escutia et al., 2011). This cyclicity documents the successive advances and retreats of the ice sheet and bounding main ice cover, too equally the varying intensity of cold saline density flows related to bottom water production at the Wilkes Country margin (due east.g., loftier-salinity shelf water flowing from the shelf into the deep sea to course Antarctic Bottom H2o, AABW) (Escutia et al., 2011). In general, typical Southern Ocean open cold-water taxa, with variable abundances of ocean water ice–associated diatoms were recovered, indicating a loftier-nutrient, high-productivity bounding main ice–influenced setting throughout the Neogene. Combined sedimentological and microfossil data indicates the ever-increasing influence of typical Antarctic Counter Current surface waters and intensifying AABW flow. Furthermore, the preservation of calcareous microfossils in several intervals indicates times when bottom waters were favorable to the preservation of calcium carbonate. These observations pointed to a very dynamic ice sheet/body of water ice regime during the late Miocene through the Pleistocene (Escutia et al., 2011).

A master focus of postcruise research to date has been the label of Pliocene and Pleistocene glacial–interglacial cycles, warm intervals, and transitions. Sediment records from Site U1361 deposited between five.3 and iii.3 meg years agone consist of alternating diatom-rich silty clay layers and diatom-poor clay layers with silt laminations (Figure iii.iii.eight(C)). Diatom-rich sediments prove a correlation with college diatom valve and bulk sediment biogenic opal concentrations, and distinctively lower signals in natural gamma radiations (Effigy iii.3.eight(C)), indicating lower clay content. The diatom-rich units also correlate with higher Ba/Al ratios (Effigy 3.iii.8(C)), suggesting multiple extended periods of increased biological productivity related to less sea ice, and warmer jump and summer bounding main surface temperatures. This is supported past work on other Antarctic margins that report increases in Southern Sea surface water productivity and sea ice loss, associated with elevated circum-Antarctic temperatures (e.g., Whitehead & Bohaty, 2003; Escutia et al., 2009; Naish et al., 2009, Effigy 3.3.eight(C)). In improver, the geochemical provenance of detrital material deposited during these warm intervals also suggests that during the warm periods there is active erosion of continental bedrock from inside the WSB, an expanse today buried below the East Antarctic Water ice Canvas. These information, also every bit the maximum modeled erosion for the northern function of the WSB (Jamieson, Sugden, & Hulton, 2010) are in agreement with retreat of the ice margin several 100 km inland (Cook et al., 2013). Such retreat could take contributed between iii and x m of global sea level rise from the East Antarctic Ice Canvass, providing a new and crucial target for future ice sail modeling. Irrespective of the extent of ice retreat, Cook et al. (2013) document a dynamic response of the E Antarctic Ice Sail to varying Pliocene climatic conditions, revealing that low-lying areas of Antarctica'south ice sheets are vulnerable to change under warmer than modernistic weather condition, with important implications for the future behavior and sensitivity of the East Antarctic Ice Canvas. Orbital calibration time-series of ice-rafted debris from Site U1361 reveal that during the early Pliocene warm conditions hateful annual isolation paced by obliquity had more than influence on Antarctic ice volume than summer insolation intensity modulated by the precession cycle (Patterson et al., 2014). A transition to precession dominance subsequently 3.5 Ma reflects a declining influence of oceanic forcing as high-latitude southern ocean cooled and perennial summertime body of water-ice field developed (Patterson et al., 2014).

The drilling of shelf Site U1358 (Figure 3.3.three) was aimed at providing a proximal tape of grounding line advances and retreats during Miocene–Pliocene times (Escutia et al., 2011). Visual core descriptions, particle size distribution, and major and trace element ratios signal that the lower Pliocene strata formed by intermittent glaciomarine sedimentation with open-marine conditions and extensive glacial advances to the outer shelf (Orejola, Passchier, & IODP Expedition 318 Scientists, 2014). In add-on, a shift in provenance is recorded by heavy mineral analyses. Sand-sized detritus in the lower Pliocene strata is sourced from local intermediate to high-form metamorphic rocks near Mertz Glacier (Orejola et al., 2014). In contrast to Pleistocene diamictons sourced from a prehnite–pumpellyite green schist facies terrane suggesting supply via iceberg rafting from northern Victoria Land (Orejola et al., 2014). This sedimentological evidence, is postulated as a shift from a dynamic EAIS margin in the early Pliocene to possible stabilization in the Pleistocene (Orejola et al., 2014).

East Antarctic Water ice sheet stability during the Pleistocene is the focus of several ongoing studies. Of these, provenance studies from the heavy mineral fraction from sediments younger than two.5 Ma recovered from Site U1359, show magmatic and metamorphic affinities that point to two sediment sources (Pant et al., 2013). A high-grade metamorphic source is consequent with the high-grade gneisses and orthogeneisses along the marginal zone of Terre Adèlie. The basaltic component in these sediments is interpreted as to exist derived from the Jurassic tholeiites in the Transantarctic Mountains (Pant et al., 2013). Study of sediments from late Miocene and early Pliocene warm events accept reported deposition of Ice Rafted Debris (IRD) sourced from the Ross Ocean region (Williams et al., 2010). This suggests that the presence of basaltic component at Site U1359, could record changes in ocean apportionment during Pleistocene warm events linked to cryosphere loss (Pant et al., 2013).

Read full chapter

URL:

https://world wide web.sciencedirect.com/scientific discipline/commodity/pii/B9780444626172000128