Pre MIS 12 glaciations in Britain and Ireland: synchronisation with other European and circum North Atlantic ice sheets, and geological controls on their preservation

The late Middle and Late Pleistocene history of the British-Irish Ice Sheet (BIIS) is well documented in both offshore and terrestrial records, and there is compelling evidence for widespread glaciations during MIS 12 (in Britain), MIS 8 (in Ireland), and within both Britain and Ireland during MIS 6 and 2. Ongoing controversies surround the existence of older ice in Ireland, and the possibility of glaciations during MIS 16 and 10 within Britain. Prior to MIS 12, the history of the BIIS is something of an enigma, especially when compared to the history of other ice sheets within Europe and the North Atlantic region. Equally, the position of Britain and Ireland relative to the Polar Front and the North Atlantic Current, and the highly dynamic nature of the ice sheet during known glaciations, make it perplexing as to why evidence for earlier glaciation is so limited. Is this enigma an artefact of stratigraphic and geological interpretation? Is it a facet of preservation linked to the scale of glaciation? Or, were Britain and Ireland simply not glaciated during this time interval? Within this paper, we examine these issues as we present, based upon new stratigraphic information, an extended glacial history of the BIIS that spans the past 2.6 Ma and demonstrates the repeated expansion of ice into marine areas prior to the Mid-Pleistocene Transition. Whilst, uncertainties exist surrounding the precise timing of many of the Early Pleistocene glacial events, at a crude temporal scale the BIIS exhibits a similar step-wise increase in the scale of glaciation displayed by other ice sheets in Europe and the North Atlantic region

Aragonite loss in a cold-water coral mound: Mechanisms and implications

Selective dissolution of aragonitic grains is emerging as a volumetrically significant process that affects a broad range of modern carbonate settings. This study explores mechanisms and implications of aragonite loss in Challenger Mound, a giant cold-water coral (Lophelia pertusa) mound of Pleistocene age, which lies on the continental slope off southwest Ireland. A comprehensive sampling scheme allowed the integration of petrographic data with geochemical analyses of sediment and pore water. The mound remains virtually unlithified and consists of stacked, fining-upward cycles of silty coral floatstone–rudstone and bafflestone grading into wackestone. Whereas calcitic grains appear unaltered, aragonitic grains are corroded and fragmented. Aragonite dissolution is attributed to organic matter oxidation at/near the sediment–water interface and, at greater depths, to the initial stages of bacterially mediated sulfate reduction, when alkalinity production is outpaced by the generation of H+. Pore water profiles indicate that undersaturated waters are diffusing towards the mound interior from two centers of sulfate reduction: one located in the upper 10 m of the sediment column and a second that lies below an erosional unconformity which marks the base of the mound. Continued aragonite dissolution is expected to gradually lower the diagenetic potential of the Challenger Mound and delay lithification until deep burial, when solution-compaction processes come into play. Despite a fundamental role in predestining the final taphonomic and textural characteristics of Challenger Mound, the processes described here are expected to leave little trace in the geological record due to a lack of cementation and calcitization. Assuming that similar processes have been active throughout the Phanerozoic, results imply that the understanding of diagenetic processes in carbonate systems may be incomplete

Sediment dynamics and palaeo-environmental context at key stages in the Challenger cold-water coral mound formation: Clues from sediment deposits at the mound base

IODP Expedition 307, targeting the 160 m high Challenger Mound and its surroundings in the Porcupine Seabight, NE Atlantic, was the first occasion of scientific drilling of a cold-water coral carbonate mound. Such mound structures are found at several locations along the continental margin but are especially numerous off Ireland. All rooted on a common unconformity (RD1) and embedded in drift sediments, the mounds in the Porcupine Seabight remain enigmatic structures, and their initial trigger and formation mechanisms are still not entirely clear. This paper discusses the sedimentary environment during the initial stages of Challenger Mound, and at the start-up of the embedding sediment drift. The results are interpreted within the regional palaeo-environmental context. Based on detailed grain-size analyses and planktonic foraminifera assemblage counts, a 14-m interval overlying the regional base-of-mound unconformity RD1 is characterised at IODP Sites U1317 (on mound), U1316 (off mound), and U1318 (background site). Several sedimentary facies are identified and interpreted in relation to regional current dynamics. Using the foraminifera counts, existing age models for the initial stages of on-mound and off-mound sedimentation are refined. Sedimentation within the initial mound was characterised by a two-mode system, with the observed cyclicities related to glacial/interglacial stages. However, the contrast in environmental conditions between the stages was less extreme than observed in the most recent glacial/interglacial cycles, allowing continuous cold-water coral growth. This sustained presence of coral framework was the key factor for fast mound build-up, baffling sediments at periods of slack currents, and protecting them from renewed erosion during high-current events. The off-mound and background sedimentation consisted mainly of a succession of contourite beds, ranging from sandy contourites in the initial stages to muddy contourites higher up in the sequence, representing the true onset of drift sedimentation. The latter illustrate the increasing importance of glacial conditions after the Mid-Pleistocene Revolution. The overall findings are summarised in a descriptive conceptual model

Precession modulation of the South Pacific westerly wind belt over the past million years.

The southern westerly wind belt (SWW) interacts with the Antarctic Circumpolar Current and strongly impacts the Southern Ocean carbon budget, and Antarctic ice-sheet dynamics across glacial-interglacial cycles. We investigated precipitation-driven sediment input changes to the Southeast Pacific off the southern margin of the Atacama Desert over the past one million years, revealing strong precession (19/23-ka) cycles. Our simulations with 2 ocean-atmosphere general circulation models suggest that observed cyclic rainfall changes are linked to meridional shifts in water vapor transport from the tropical Pacific toward the southern Atacama Desert. These changes reflect a precessional modulation of the split in the austral winter South Pacific jet stream. For precession maxima, we infer significantly enhanced rainfall in the southern Atacama Desert due to a stronger South Pacific split jet with enhanced subtropical/subpolar jets, and a weaker midlatitude jet. Conversely, we derive dry conditions in northern Chile related to reduced subtropical/subpolar jets and an enhanced midlatitude jet for precession minima. The presence of precessional cycles in the Pacific SWW, and lack thereof in other basins, indicate that orbital-scale changes of the SWW were not zonally homogeneous across the Southern Hemisphere, in contrast to the hemispherewide shifts of the SWW suggested for glacial terminations. The strengthening of the jet is unique to the South Pacific realm and might have affected winter-controlled changes in the mixed layer depth, the formation of intermediate water, and the buildup of sea-ice around Antarctica, with implications for the global overturning circulation and the oceanic storage of atmospheric CO2

The importance of the terrigenous fraction within a cold-water coral mound: A case study

In the nineties, cold-water coral mounds were discovered in the Porcupine Seabight (NE Atlantic, west of Ireland). A decade later, this discovery led to the drilling of the entire Challenger cold-water coral mound (Eastern slope, Porcupine Seabight) during IODP Expedition 307. As more than 50% of the sediment within Challenger Mound consists of terrigenous material, the terrigenous component is equally important for the build-up of the mound as the framework-building corals. Moreover, the terrigenous fraction contains important information on the dynamics and the conditions of the depositional environment during mound development. In this study, the first in-depth investigation of the terrigenous sediment fraction of a cold-water coral mound is performed, combining clay mineralogy, sedimentology, petrography and Sr–Nd-isotopic analysis on a gravity core (MD01-2451G) collected at the top of Challenger Mound.Sr- and Nd-isotopic fingerprinting identifies Ireland as the main contributor of terrigenous material in Challenger Mound. Besides this, a variable input of volcanic material from the northern volcanic provinces (Iceland and/or the NW British Isles) is recognized in most of the samples. This volcanic material was most likely transported to Challenger Mound during cold climatic stages. In three samples, the isotopic ratios indicate a minor contribution of sediment deriving from the old cratons on Greenland, Scandinavia or Canada. The grain-size distributions of glacial sediments demonstrate that ice-rafted debris was deposited with little or no sorting, indicating a slow bottom-current regime. In contrast, interglacial intervals contain strongly current-sorted sediments, including reworked glacio-marine grains. The micro textures of the quartz-sand grains confirm the presence of grains transported by icebergs in interglacial intervals. These observations highlight the role of ice-rafting as an important transport mechanism of terrigenous material towards the mound during the Late Quaternary.Furthermore, elevated smectite content in the siliciclastic, glaciomarine sediment intervals is linked to the deglaciation history of the British-Irish Ice Sheet (BIIS). The increase of smectite is attributed to the initial stage of chemical weathering processes, which became activated following glacial retreat and the onset of warmer climatic conditions. During these deglaciations a significant change in the signature of the detrital fraction and a lack of coral growth is observed. Therefore, we postulate that the deglaciation of the BIIS has an important effect on mound growth. It can seriously alter the hydrography, nutrient supply and sedimentation processes, thereby affecting both sediment input and coral growth and hence, coral mound development

(Table 1) Neodymium and Strontium measurements from sediment core MD01-2451

In the nineties, cold-water coral mounds were discovered in the Porcupine Seabight (NE Atlantic, west of Ireland). A decade later, this discovery led to the drilling of the entire Challenger cold-water coral mound (Eastern slope, Porcupine Seabight) during IODP Expedition 307. As more than 50% of the sediment within Challenger Mound consists of terrigenous material, the terrigenous component is equally important for the build-up of the mound as the framework-building corals. Moreover, the terrigenous fraction contains important information on the dynamics and the conditions of the depositional environment during mound development. In this study, the first in-depth investigation of the terrigenous sediment fraction of a cold-water coral mound is performed, combining clay mineralogy, sedimentology, petrography and Sr-Nd-isotopic analysis on a gravity core (MD01-2451G) collected at the top of Challenger Mound. Sr- and Nd-isotopic fingerprinting identifies Ireland as the main contributor of terrigenous material in Challenger Mound. Besides this, a variable input of volcanic material from the northern volcanic provinces (Iceland and/or the NW British Isles) is recognized in most of the samples. This volcanic material was most likely transported to Challenger Mound during cold climatic stages. In three samples, the isotopic ratios indicate a minor contribution of sediment deriving from the old cratons on Greenland, Scandinavia or Canada. The grain-size distributions of glacial sediments demonstrate that ice-rafted debris was deposited with little or no sorting, indicating a slow bottom-current regime. In contrast, interglacial intervals contain strongly current-sorted sediments, including reworked glacio-marine grains. The micro textures of the quartz-sand grains confirm the presence of grains transported by icebergs in interglacial intervals. These observations highlight the role of ice-rafting as an important transport mechanism of terrigenous material towards the mound during the Late Quaternary. Furthermore, elevated smectite content in the siliciclastic, glaciomarine sediment intervals is linked to the deglaciation history of the British-Irish Ice Sheet (BIIS). The increase of smectite is attributed to the initial stage of chemical weathering processes, which became activated following glacial retreat and the onset of warmer climatic conditions. During these deglaciations a significant change in the signature of the detrital fraction and a lack of coral growth is observed. Therefore, we postulate that the deglaciation of the BIIS has an important effect on mound growth. It can seriously alter the hydrography, nutrient supply and sedimentation processes, thereby affecting both sediment input and coral growth and hence, coral mound development

Precession modulation of the South Pacific westerly wind belt over the past million years

The southern westerly wind belt (SWW) interacts with the Antarctic Circumpolar Current and strongly impacts the Southern Ocean carbon budget, and Antarctic ice-sheet dynamics across glacial- interglacial cycles. We investigated precipitation-driven sediment input changes to the Southeast Pacific off the southern margin of the Atacama Desert in Chile over the past one million years, revealing strong precession (19/23-ka) cycles. Our simulations with 2 ocean-atmosphere general circulation models suggest that observed cyclic rainfall changes are linked to meridional shifts in water vapor transport from the tropical Pacific toward the southern Atacama Desert. These changes reflect a precessional modulation of the split in the austral winter South Pacific jet stream. For precession maxima, we infer significantly enhanced rainfall in the southern Atacama Desert due to a stronger South Pacific split jet with enhanced subtropical/subpolar jets, and a weakermidlatitude jet. Conversely, we derive dry conditions in northern Chile related to reduced subtropical/subpolar jets and an enhanced midlatitude jet for precession minima. The presence of precessional cycles in the Pacific SWW, and lack thereof in other basins, indicate that orbital-scale changes of the SWW were not zonally homogeneous across the Southern Hemisphere, in contrast to the hemispherewide shifts of the SWW suggested for glacial terminations. The strengthening of the jet is unique to the South Pacific realm and might have affected winter-controlled changes in the mixed layer depth, the formation of intermediate water, and the built-up of sea-ice around Antarctica, with implications for the global overturning circulation and the oceanic storage of atmospheric CO2

Cold-water carbonate mounds as palaeo-archives : the Plio-Pleistocene sediment record from the Challenger Mound, northeast Atlantic

During IODP Expedition 307, the first (and so far only) complete sequence through a cold-water coral carbonate mound was successfully drilled. After decades of research on contemporary to sub-recent coral carbonate mound environments, the complete recovery of the Challenger Mound record, sampling one of the large mounds along the Irish sector of the NE Atlantic continental margin (eastern Porcupine Seabight; Belgica mound province), allowed for the first time the investigation of long-term mound development, from mound initiation to decline. Here, we present an overview of the palaeo-environmental (i.e. hydrodynamic, oceanographic and climatic) signal captured in the entire Challenger Mound sequence (Hole U1317E). A high-resolution multi-proxy characterisation of the ca. 155m long mound matrix sediment record was conducted, encompassing a wide array of sedimentological, mineralogical, geochemical and stratigraphic techniques. These included, amongst others, siliciclastic particle-size analysis, X-ray diffraction phase quantification, isotopic fingerprinting of target elements for provenance purposes, calcareous nannofossil and planktonic foraminifer biostratigraphy and assemblage counts. In this way we aimed to (1) identify the controls on Challenger Mound development throughout the different phases of its entire Plio-Pleistocene to recent build-up, and (2) assess the unique character of coral carbonate mounds as recorders of Quaternary palaeo-environmental change at intermediate water depth in the NE Atlantic. Overall, Challenger Mound development shows a strong affinity to the general climate variability of the Northern Hemisphere, although not being completely in phase with it. The major oceanographic and climatic rearrangements of the Plio-Pleistocene, such as those associated with the Late Pliocene/Early Pleistocene intensification of continental ice-sheet development (ca. 2.75 – 2.55 Ma)1 or the orbital frequency changes during the mid-Pleistocene climate transition (ca. 1.2 – 0.6 Ma)2, seem responsible for the two significant thresholds in Challenger Mound development: its Late Pliocene origin and its Middle-Late Pleistocene to recent decline. However, local influences such as proximal (British-Irish) ice-sheet dynamics and on-mound changes in cold-water coral density seem to have a stronger control on Challenger Mound development and may have induced the offset between global climate and Challenger Mound proxy record variability. On the other hand, owing to this, a unique, high-resolution palaeo-record of regional Early-Pleistocene environmental change (including early British-Irish ice-sheet development3) is preserved in the lower Challenger Mound, covering a period that is not recorded in the general sedimentary sequence of the area. The Challenger Mound record, albeit with restricted Late Quaternary preservation, highlights the potential of coral carbonate mounds as excellent palaeo-recorders, providing us with unique records from ‘complex’ continental margin environments