Aggradation and carbonate accumulation of Holocene Norwegian cold‐water coral reefs

Abstract

Cold‐water coral ecosystems present common carbonate factories along the Atlantic continental margins, where they can form large reef structures. There is increasing knowledge on their ecology, molecular genetics, environmental controls and threats available. However, information on their carbo‐nate production and accumulation is still very limited, even though this information is essential for their evaluation as carbonate sinks. The aim of this study is to provide high‐resolution reef aggradation and carbonate accumulation rates for Norwegian cold‐water coral reefs from various settings (sunds, inner shelf and shelf margin). Furthermore, it introduces a new approach for the evaluation of the cold‐water coral preservation within cold‐water coral deposits by computed tomography analysis. This approach allows the differentiation of various kinds of cold‐water coral deposits by their macrofossil clast size and orientation signature. The obtained results suggest that preservation of cold‐water coral frameworks in living position is favoured by high reef aggradation rates, while preservation of coral rubble prevails by moderate aggradation rates. A high degree of macrofossil fragmentation indicates condensed intervals or unconformities. The observed aggradation rates with up to 1500 cm kyr−1 exhibit the highest rates from cold‐water coral reefs so far. Reef aggradation within the studied cores was restricted to the Early and Late Holocene. Available datings of Norwegian cold‐water corals support this age pattern for other fjords while, on the shelf, cold‐water coral ages are reported additionally from the early Middle Holocene. The obtained mean carbonate accumulation rates of up to 103 g cm−2 kyr−1 exceed previous estimates of cold‐water coral reefs by a factor of two to three and by almost one order of magnitude to adjacent sedimentary environments (shelf, slope and deep sea). Only fjord basins locally exhibit carbonate accumulation rates in the range of the cold‐water coral reefs. Furthermore, cold‐water coral reef carbonate accumulation rates are in the range of tropical reef carbonate accumulation rates. These results clearly suggest the importance of cold‐water coral reefs as local, maybe regional to global, carbonate sinks.

Mediterranean cold‐water corals – an important regional carbonate factory?

“…; Table S1; Dorschel et al ., ; Titschack et al ., ) while the rates obtained for the SML coral province are in the same range. The CARs of the Mediterranean CWC sites are in the lower range of the prolific Norwegian CWC occurrences (12 to 2114 g cm −2 kyr −1 ; Titschack et al ., ). For example, the maximal CAR of up to 502 g cm −2 kyr −1 identified for the eastern Melilla coral province is two to four times smaller than the maximal CARs known from Norwegian CWC reefs.…”

“…Within the Mediterranean CWC deposits studied here, the CT analyses allowed the differentiation of three distinct preservation patterns that are generally comparable to the interpretations of Titschack et al . () from Norwegian CWC sites. The first pattern is characterized by CWC clasts with predominantly large clast sizes (mean: <−5.0 Ф) and clast orientations of up to 90° (vertical) with well‐defined clast orientation maxima that are associated with aggradation rates exceeding ca 300 cm kyr −1 .…”

“…The deposits are interpreted to have been accumulated by the gravitational, most‐probably fall‐like, down‐cliff transport of CWC clasts and hemipelagic sediments, both forming a talus deposit. This process of formation is also reflected in a significantly different preservation pattern compared to all other deposits presented here and known from Norwegian CWC sites (Titschack et al ., ). The CWC‐bearing units of the core from Urania Bank (Units A and C in core GeoB11135‐2; Fig.…”

“…Therefore, the methodological approach follows Titschack et al . () and combines the CWC content, determined from CT image data (every 0.3 mm), with XRD analysis of the matrix sediment (see also Titschack et al ., ). This approach allowed a further differentiation of the carbonate sources – CWC‐carbonate factory‐derived (subsequently abbreviated as CWC‐fac.‐derived ) versus background‐sediment‐derived (subsequently abbreviated as backgr.‐sed.‐derived ) carbonate.…”

“…However, new results from CWC deposits off Norway with mean CARs of about 100 g cm −2 kyr −1 (max. of up to 2100 g cm −2 kyr −1 ) suggest that CWC systems have the potential to reach CARs that are in the range of tropical shallow‐water reefs (Titschack et al ., ).…”