Environmental boundary conditions of cold-water coral mound growth over the last 3 million years in the Porcupine Seabight, Northeast Atlantic

Paleoenvironmental reconstruction of Challenger Mound initiation in the Porcupine Seabight, NE Atlantic

Modern cold-water corals (CWCs) occur in a wide range of water depths, with Desmophyllum pertusum being one of the most common species. Pleistocene, Holocene, and modern coral mound formation by living CWC reefs have previously been described in the Porcupine Seabight from water depths greater than 700 m in the vicinity of the transitional zone between the Eastern North Atlantic Water and Mediterranean Outflow Water. Here we document occurrence of fossil corals retrieved from two cores at 370 m depth in the Macnas Mounds, a relatively shallow occurrence for mounds on the Irish shelf-edge. Both cores feature D. pertusum restricted to the upper two metres, immediately overlying an erosive surface and a coeval major down-core change in grain size from sand to mud. Radiocarbon dating of coral specimens indicates the CWC mounds initiated 7.82 Cal ky BP. Our study unequivocally documents the existence of Holocene shelf-edge coral mounds in the eastern Porcupine Seabight and highlights the possibility of other occurrences of CWCs in similar settings elsewhere in the northeast Atlantic. Given that no living CWCs were encountered in the study area, we suggest that the area previously experienced more favourable conditions for CWC mound initiation and development along the shelf-edge margin, possibly due to differing conditions in the European Slope Current which flows northward along the continental slope from south of the Porcupine Bank to the Faroe-Shetland Channel.Graphical

Deglacial upslope shift of NE Atlantic intermediate waters controlled slope erosion and cold-water coral mound formation (Porcupine Seabight, Irish margin)

The Moira Mounds, small cold-water coral mounds in the Porcupine Seabight, NE Atlantic: Part B—Evaluating the impact of sediment dynamics through high-resolution ROV-borne bathymetric mapping

U‐Th ages and temperatures derived from Li/Mg have been measured on coral fragments of Lophelia pertusa and Madrepora oculata collected from two sediment cores, which were taken from cold‐water coral (CWC) mounds at 700–790m water depth at the SW Rockall Trough margin. Our data, combined with previous published data, have allowed us to first estimate the occurrence of CWC at the SW Rockall Trough margin during the Holocene and, second, to better constrain the environmental conditions driving variability in CWC growth. CWC abundance is marked by a pronounced increase in the mid‐Holocene (∼6 ka) and is modulated by millennial‐scale variability throughout the late‐Holocene. The mid‐Holocene proliferation of CWC coincides with lowest IRD abundances and a major reorganization of the circulation at thermocline depth in the Rockall Trough, marked by the progressive replacement of the fresh‐cold Sub‐Arctic Intermediate Water (SAIW) by the saltier and nutrient‐rich Eastern North Atlantic Water (ENAW). This event must have established a modern‐like winter mixed layer and thermocline structure, generating suitable conditions for enhanced surface productivity, downslope transport of food particles, bottom current acceleration at mound depth and thus CWC growth. Several short time intervals of decreased CWC occurrences closely match prominent increases in North Atlantic drift ice and storminess in Northern Europe. We, therefore, propose that high detrital supply and/or changes in the vertical density gradient associated with millennial‐scale ice‐rafted detritus (IRD) events are the likely controlling factors for CWC growth and subsequent mound formation on the SW Rockall Trough margin.

Nd isotopes in deep-sea corals in the North-eastern Atlantic

Stable isotopes and element compositions of the fine‐grained matrix were measured for IODP Expedition 307 Hole U1317E drilled from the summit of Challenger Mound in Porcupine Seabight, northeast Atlantic, to explore the palaeoceanographic and palaeoclimatic background to development of the deep‐water coral mound. The 155 m long mound section was divided into two units by an unconformity at 23.6 mbsf: Unit M1 (2.6–1.7 Ma) and Unit M2 (1.0–0.5 Ma). Results from 519 specimens show a difference in δ13C value between Unit M1 (−0.6‰ to −5.0‰) and Unit M2 (−1.0‰ to 1.0‰), but such a distinct difference was not seen in δ18O values (1.0‰–2.5‰), CaCO3 content (40–60 wt%), Sr/Ca ratio (2.0–8.0 mmol mol−1), and Mg/Ca ratio (10.0–20.0 mmol mol−1) through the mound. Positive δ18O and negative δ13C shifts at the mound base are consistent with the oceanographic changes in the northeast Atlantic at the beginning of the Quaternary. The positive δ13C regression in Unit M2 suggests a linkage to the mid Pleistocene intensified glaciation in the Northern Hemisphere. Warm Mediterranean Upper Core Water of Mediterranean Outflow Water, Eastern North Atlantic Water and cold Labrador Sea Water of North Atlantic Deep Water are key oceanographic features that cause spikes and shifts in stable isotope and element composition. However, the stable isotope values of the sediment matrix could not primarily record the glacial–interglacial eustatic/temperature change, but indirectly indicate current regimes of the intermediate oceanic layer where the coral mound grew. Similarly, elemental ratios and CaCO3 content may not represent the productivity and temperature of surface sea water, respectively, but superpose the fractions from both surface and bottom water. It is concluded that palaeoceanographic change coupled to the Pleistocene glacial/interglacial cycles is a key control on the geochemical stratigraphy of the matrix sediments of the carbonate mound developed in Porcupine Seabight. Copyright © 2011 John Wiley & Sons, Ltd.

Sedimentary processes and cold-water coral mini-mounds at the Ferrol canyon head, NW Iberian margin

<p>Mediterranean Outflow Water (MOW) acts as a net source of salt and heat into North Atlantic intermediate depths that ultimately contributes to the Atlantic Meridional Overturning Circulation. On this basis, it has been hypothesised that MOW variability might influence global climate. Although several studies have documented major glacial-interglacial changes in deep- and intermediate Mediterranean circulation patterns, little is known about associated impacts on MOW properties, in particular its residence time and geochemical signature. Using a set of cold-water coral samples from along the ‘pre-MOW’ and MOW path, i.e. from the Alboran Sea to the northern Galician Bank including the Strait of Gibraltar and the Gulf of Cadiz, we aim to identify changes in both the ventilation state of the water masses flowing out of the Mediterranean and the distribution of coral growth.With this purpose, paired Uranium-series and AMS radiocarbon ages have been obtained in the same coral samples allowing any potential change in the reservoir age to be inferred accurately, as well as allowing a spatio-temporal ‘coral map’ to be created. Furthermore, these results have been complemented by trace element measurements in benthic foraminifera from the Alboran coral mound sediment core.</p><p>Our results show a particular spatio-temporal coral distribution with glacial presence only at the deepest sites of the Gulf of Cadiz (~1000m), followed by ~300m Western Mediterranean (WMed) coral appearance across the deglaciation/mid Holocene (14-4 kyr), to end with a proliferation at the Strait of Gibraltar and Galicia Bank from ~6 kyr towards the present. We hypothesise 1) that ~300m WMed area might have been bathed in Atlantic waters inflow during the glacial due to sea-level drop, returning to LIW (Levantine Intermediate Water) influence over the deglaciation, and 2) that MOW reached deeper areas outside of the Mediterranean Sea in the Gulf of Cadiz during the glacial period. Regarding the reservoir age, little change at the WMed is observed at 150-450m across the deglaciation as compared to the large ventilation excursion detected in the Iberian Margin at ~1000m. However, a ventilation age gradient of ~300 yr related to water depth is observed within WMed corals when appearing at the Bølling-Allerød, in synchrony with significant changes in hydrographical parameters inferred from foraminiferal trace element from the same area. Overall, our results suggest a water mass reorganization at the surface-intermediate layer of the WMed during the deglaciation and early Holocene, but the ultimate nature of these changes needs yet to be explored by further analysis of Nd isotopes as well as of trace elements beyond the deglaciation.</p>

Water masses in the upper and middle North Atlantic Ocean east of the Azores

Hydrodynamic controls on cold-water coral growth and carbonate-mound development at the SW and SE Rockall Trough Margin, NE Atlantic Ocean

During the last 15 years, occurrences of large mound clusters have increasingly been reported along the northeastern Atlantic margins. These mound structures are in many cases covered with cold-water corals, suggesting an enigmatic relationship between cold-water coral growth and the development of such mounds. The growth of cold-water corals and the origin of recent deep-water carbonate mounds was a heavily debated subject during the last decade. Different theories were invoked concerning cold-water coral growth and the development of carbonate mounds. Two main schools developed to explain the origin of recent cold-water coral reefs and carbonate mounds. One school is mainly based on internal controls whereby light hydrocarbon seepage would play an important role in the initial phase of reef and mound growth (Hovland 1990, 1994, 1998; Hovland and Thomsen 1997; Henriet et al. 1998, 2001). The second school relies on the impact of external and environmental controls, such as favourable oceanographic conditions, on cold-water coral growth and so mound build-up (Freiwald et al. 1997, 1999; Freiwald and Wilson 1998; Mortensen 2000; De Mol et al. 2002; Duineveld et al. 2004).

Environmental setting of deep-water oysters in the Bay of Biscay

θ-S relationships and water masses in the eastern North Atlantic

Mediterranean-Atlantic water exchange over the Miocene-Pliocene boundary