Paleoseawater density reconstruction and its implication for cold‐water coral carbonate mounds in the northeast Atlantic through time

Abstract

Carbonate buildups and mounds are impressive biogenic structures throughout Earth history.In the recent NE Atlantic, cold-water coral (CWC) reefs form giant carbonate mounds of up to 300 m of elevation. The expansion of these coral carbonate mounds is paced by climatic changes during the past 2.7 Myr. Environmental control on their development is directly linked to controls on its main constructors, the reef-building CWCs. Seawater density has been identified as one of the main controlling parameter of CWC growth in the NE Atlantic. One possibility is the formation of a pycnocline above the carbonate mounds, which is increasing the hydrodynamic regime, supporting elevated food supply, and possibly facilitating the distribution of coral larvae. The potential to reconstruct past seawater densities from stable oxygen isotopes of benthic foraminifera has been further developed: a regional equation gives reliable results for three different settings, peak interglacials (e.g., Holocene), peak glacials (e.g., Last Glacial Maximum), and intermediate setting (between the two extremes). Seawater densities are reconstructed for two different NE Atlantic CWC carbonate mounds in the Porcupine Seabight indicating that the development of carbonate mounds is predominantly found at a seawater density range between 27.3 and 27.7 kg m À3 (σ Θ notation). Comparable to recent conditions, we interpret the reconstructed density range as a pycnocline serving as boundary layer, on which currents develop, carrying nutrition and possibly coral larvae. The close correlation of CWC reef growth with reconstructed seawater densities through the Pleistocene highlights the importance of pycnoclines and intermediate water mass dynamics.

Solenosmilia variabilis-bearing cold-water coral mounds off Brazil

“…6; Rohling et al 2014) implying a deepening of the water-mass boundary between the SACW and the AAIW. Such a water-mass stratification may favour the supply of particulate organic matter (POM) from high seasurface productivity settling at the pycnocline or watermass boundaries and then getting distributed by strong internal waves or along-slope bottom currents (Mienis et al 2007;White et al 2007;Rüggeberg et al 2016). On the Brazilian shelf the organic-matter (OM) transport is controlled largely by sea-level changes (Albuquerque et al 2016) and is locally enhanced due to strong sea-surface productivity (see below).…”

Environmental constraints on Holocene cold‐water coral reef growth off Norway: Insights from a multiproxy approach

“…On geological time scales the initiation, development, and demise of CWC reefs have been sensitive to various oceanographic parameters such as bottom currents, nutrient availability/sea surface productivity, and water mass stratification [e.g., Eisele et al ., ; Hebbeln et al ., ; Kano et al , ; Raddatz et al ., , ; Rüggeberg et al ., , ]. Particularly, CWC growth in the NE Atlantic reveals a distinct shift of the biogeographic limit from high to low latitudes in glacial periods [ Eisele et al ., ; Frank et al ., , , ; McCulloch et al ., ; Wienberg et al ., , ].…”

“…Particularly, CWC growth in the NE Atlantic reveals a distinct shift of the biogeographic limit from high to low latitudes in glacial periods [ Eisele et al ., ; Frank et al ., , , ; McCulloch et al ., ; Wienberg et al ., , ]. During interglacial periods (Holocene) CWC reef growth was predominantly restricted to the Norwegian and Irish margins as well as to the Bay of Biscay [ Eisele et al ., ; Dorschel et al ., ; Rüggeberg et al ., , ; Frank et al ., ], whereas CWC reef growth off Mauretania and in the Gulf of Cadiz occurred mostly during glacial periods [ Eisele et al ., ; Frank et al ., ; Wienberg et al ., ]. Such an obviously climatically driven pattern of active CWC reef growth has been primarily attributed to changes in sea surface productivity and movements of the Polar Front [ Frank et al ., , and references therein].…”

Monsoonal forcing controlled cold water coral growth off south-eastern Brazil during the past 160 kyrs

“…Mound lies within the nutrient-rich AAIW, which might have boosted CWC growth at the East Brazilian slope. Second, 320 nutrients and POC typically concentrate within nepheloid layers at water mass boundaries and provide a prolific food source for CWCs (Mienis et al, 2007;Dullo et al, 2008;Raddatz et al, 2014;Rüggeberg et al, 2016;Magill et al, 2018). In the case of Bowie Mound it might thus be suspected that enhanced production (Pahnke and Zahn, 2005;Pahnke et al, 2008) and/or nutrient-enrichment (i.e.…”

“…All affect the capacity of CWC to build their aragonitic framework (e.g., Form and Riebesell, 2012;Maier et al, 2012;Lunden et al, 2014;Hennige et al, 2015;Büscher et al, 2017). Spatial fluctuations of intermediateto deep-water masses further influence the depth and strength of pycnoclines, which are thought to play an important 60 role for the concentration and dispersal of nutrients and food utilized by CWC (Frederiksen et al, 1992;Mienis et al, 2007;Rüggeberg et al, 2016). Aside of processes directly affecting the water-mass properties bathing CWC, several studies also point at the importance of surface productivity in providing food that is transported to the deep ocean (Davies et al, 2009;Soetaert et al, 2016).…”