The Importance of Ecological Accommodation Space and Sediment Supply for Cold-Water Coral Mound Formation, a Case Study From the Western Mediterranean Sea

Abstract

The formation of cold-water coral (CWC) mounds is commonly seen as being the result of the sustained growth of framework-forming CWCs and the concurrent supply and deposition of terrigenous sediments under energetic hydrodynamic conditions. Yet only a limited number of studies investigated the complex interplay of the various hydrodynamic, sedimentological and biological processes involved in mound formation, which, however, focused on the environmental conditions promoting coral growth. Therefore, we are still lacking an in-depth understanding of the processes allowing the on-mound deposition of hemipelagic sediments, which contribute to two thirds of coral mound deposits. To investigate these processes over geological time and to evaluate their contribution to coral mound formation, we reconstructed changes in sediment transport and deposition by comparing sedimentological parameters (grain-size distribution, sediment composition, accumulation rates) of two sediment cores collected from a Mediterranean coral mound and the adjacent seafloor (off-mound). Our results showed that under a turbulent hydrodynamic regime promoting coral growth during the Early Holocene, the deposition of fine siliciclastic sediments shifted from the open seafloor to the coral mounds. This led to a high average mound aggradation rate of >130 cm kyr–1, while sedimentation rates in the adjacent off-mound area at the same time did not exceed 10 cm kyr–1. Thereby, the baffling of suspended sediments by the coral framework and their deposition within the ecological accommodation space provided by the corals seem to be key processes for mound formation. Although, it is commonly accepted that these processes play important roles in various sedimentary environments, our study provided for the first time, core-based empirical data proving the efficiency of these processes in coral mound environment. In addition, our approach to compare the grain-size distribution of the siliciclastic sediments deposited concurrently on a coral mound and on the adjacent seafloor allowed us to investigate the integrated influence of coral mound morphology and coral framework on the mound formation process. Based on these results, this study provides the first conceptual model for coral mound formation by applying sequence stratigraphic concepts, which highlights the interplay of the coral-framework baffling capacity, coral-derived ecological accommodation space and sediment supply.

Major environmental drivers determining life and death of cold-water corals through time

“…Of all paleoenvironmental parameters considered here, only the grain-size data used as a proxy for the hydrodynamic setting can be directly affected by the distance between the off-mound site and the coral mound site. However, even though coral mounds (being able to accelerate local currents due to their elevated topography, e.g., [ 62 ]) and thriving coral reefs on their top (being able to decelerate local currents due to the baffling effect of the coral framework, e.g., [ 63 ]) may induce local effects, their ability to modulate the regional current regime is limited. Hence, the regional hydrodynamic setting is best recorded in the off-mound records that are generally unaffected by local coral mound-related effects.…”

Morphosedimentary, Structural and Benthic Characterization of Carbonate Mound Fields on the Upper Continental Slope of the Northern Alboran Sea (Western Mediterranean)

“…Indeed, significant environmental changes on the continental slope decrease onshore sediment inputs to the continental slope due to landward migration of the coastline [6,92,93]. Hence, the mounds could be controlled by changes in the sediment focusing, resulting in lateral sediment supply and changing deposition on the mounds, as supported in [53] and also changes in the current bottom-current regimes. Considering all these observations, it is possible that changes in the water mass circulation and water column structure, productivity, near-bottom currents and sediment supply may also have induced the demise of the studied northern Alboran Sea mounds as detected in the southern Alboran Sea CWC mounds.…”

“…The bottom sediments and associated benthic communities on the mounds are similar to those detected on the seafloor adjacent to the mounds, indicating a very low present-day benthic heterogeneity between the studied mounds and the surrounding seabed. Generally, mound formation requires mound growth to be faster than background sedimentation, and baffling of hemipelagic sediment is a major component contributing to mound growth (especially in branching mound-forming organisms; [5,18,20,33,34,53]. Thus, the burial of carbonate mounds appears to be a consequence of the cessation of active reefs and, thus, mound growth induced by the deterioration of living conditions for the respective mound-forming organisms [5].…”

“…Bottom currents can also create small erosional and depositional sedimentary structures and modulate the shape of the mound [47]), resulting in flattened top mounds (within the storm wave base), cone-shaped mounds (quiet and deeper waters) [4], elongated mound shapes (due to the preferred growth of the reef-forming organisms towards the main current) [48]), and mounds with current scours and moats around them (in strong bottom deep water along slope bottom currents) [35,41,[49][50][51]. Active CWC mounds are covered by thriving CWC reefs and background sedimentation rate is generally lower than CWCs growth rate, however, contouritic and/or hemipelagic sediments have generally been found as major structural components contributing to the accretion of the CWC mounds (sometimes representing up to two-thirds of the mound deposits) [18,52,53]. Favorable environmental conditions for CWC growth or increases of background sedimentation in specific areas or time periods (e.g., sediment outputs from meltwater during deglaciation, changes in bottom current regimes, sediment extrusion in mud volcanoes, etc.)…”

A 300 000-year record of cold-water coral mound build-up at the East Melilla Coral Province (SE Alboran Sea, western Mediterranean)

“…Within the EMCP, the localities of Brittlestar Ridge I (BRI) and Dragon Mound have received the most attention during the last decade (Fig. 1b and c; Fink et al, 2013Fink et al, , 2015Stalder et al, 2015Stalder et al, , 2018Terhzaz et al, 2018;Fentimen et al, 2020a;Krengel, 2020;Rachid et al, 2020;Wang et al, 2021). U-series dating of corals revealed that the formation of Dragon Mound began 450 kyr ago, whereas BRI started building up over 538 kyr ago (Krengel, 2020).…”