Pockmark-associated coral reefs at the Kristin field off Mid-Norway

Abstract

Topographic and geophysical seafloor mapping has shown that in some areas there apparently is a close relationship between pockmarks and coral reefs. Off Norway there are three such occurrences: The Haltenpipe Reef Cluster (HRC), the Fugloy Reefs, and reefs in the Kristin hydrocarbon field. In the Porcupine area off Ireland, there is also a close relationship between large pockmark-like depressions and carbonate mounds with corals. Such is also the case with the Darwin Mounds, at 1000 m depth, where pockmarks occur close to the mounds containing Lophelia corals. In the Gulf of Mexico and off Brazil there are similar associations. At the Kristin hydrocarbon field off Mid-Norway (located 130 km NW of the HRC and the Sula Ridge Reefs), pockmarks and coral reefs occur in abundance. Out of 120 reefs mapped within an area of 14 km(2), there are 33 reefs that occur inside or along the inside rim of pockmarks. Most of the other 87 reefs are located within 200 m of a pockmark. The largest reefs at Kristin are up to 3.5 m high and 90 m in length. The observed co-occurrence between corals and pockmarks suggests that gas and porewater emanating from pockmarks stimulates coral growth, probably through the provision into the water column of a stable food (nutrient) supply, in the form of bacteria and micro-organisms. This theory has previously been called the "hydraulic theory" for coral reefs. Both of the new (Kristin and Fugloy) reef/pockmark cases strengthen the validity of this theory. Furthermore, this close association also indicates that the sessile animals living in these reefs (Lophelia, Paragorgia, Primnoa, Acesta, etc.) may be more robust and tolerant to episodes of silting than previously suspected.