Reconstruction of the formation history of the Darwin Mounds, N Rockall Trough: How the dynamics of a sandy contourite affected cold-water coral growth

Lissette Victorero,Dominique Blamart,Edwige Pons-Branchu,Mark N Mavrogordato,Veerle A.I Huvenne

doi:10.1016/j.margeo.2015.12.001

Abstract

Cold-water coral mounds, formed through a feed-back process of cold-water coral growth and sediment baffling, have been studied all along the NE Atlantic continental margin. However, major questions remain concerning their formation history, especially their initiation and early development in relation to the surrounding sediment dynamics. For the first time, two small mounds located in a sandy contourite have been cored from the top to mound base: here, the formation history of the Darwin Mounds, located in the Northern Rockall Trough was investigated and reconstructed from two piston cores using a multidisciplinary approach. This consisted of CT-scanning for quantifying coral density changes with depth, grain-size analysis to obtain the hydrodynamic trends and radiocarbon and U-series dating to place the results into a wider paleoceanographic context. The results show that the Darwin Mounds formed during the early Holocene (~10ka BP) through sediment baffling, mainly by Lophelia pertusa. The initiation of both mounds shows a similar pattern of increased current velocities resulting in coarser sediment deposition and a relatively high coral density with a peak of 23 vol%. The mound growth was rapid between ~10–9.7ka BP (up to 277cmka−1 in one of the mounds), with further vibrant growth periods around ~8.8ka BP, 6.5ka BP and 3.4ka BP. The demise of the mounds ca. ~3ka BP was likely caused by an intensification in bottom current velocities causing a hostile environment for coral growth in the contourite setting. In a wider context, the development of the Darwin Mounds appears to have responded to the relative strength and position of the Subpolar Gyre, which affected food supply to the corals, sedimentation rates, current speeds and other water mass properties in the area.

Highlights

Cold-water corals (CWC) are azooxanthellates (i.e coral species lacking photosynthesizing dinoflagellates) that thrive in water depths beyond the photic zone
From the Computerized Tomography (CT)-scan image data it is clear that the main coral species participating in the formation of the Darwin Mounds was L. pertusa
These estimates exceed those from large CWC mounds on Rockall Bank with Vertical Mound Growth Rates (VMGR) rates of 60 cm ka−1, but are in agreement with Porcupine Seabight values (220 cm ka−1, between 9.8–8.5 ka BP) (Frank et al, 2009) with such differences attributed to the sediment input being terrigenous at the Porcupine Seabight, while Rockall Bank is carbonate dominated (Van der Land et al, 2014)

Summary

Introduction

Cold-water corals (CWC) are azooxanthellates (i.e coral species lacking photosynthesizing dinoflagellates) that thrive in water depths beyond the photic zone. The most intensely studied CWC species are the scleractinians Lophelia pertusa and Madrepora oculata. The distribution of these species is associated with a) increased surface primary productivity (Van Oevelen et al, 2009), b) water temperatures between 4 and 14 °C (Roberts et al, 2006; Freiwald, 2009), c) an aragonite saturation Ωaragonite N 1 (Davies and Guinotte, 2011), d) a narrow potential density envelope (27.35–27.65 kg m3) in the North-Atlantic (Dullo et al, 2008) and e) the correct substrate for larval settlement. ⁎ Corresponding author at: Ocean & Earth Science, University of Southampton, European Way, Southampton, UK Both species thrive in moderately high energy environments where they are supplied with food particles by a variety of hydrographies, such as currents, internal tides and downwelling flows (White, 2007; Davies et al, 2009). Over geological timescales cyclical reef growth combined with sedimentation and erosion can lead to CWC mound formation (Dorschel et al, 2007a; De Haas et al, 2009)

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Conclusion