Enigmatic Deep-Water Mounds on the Orphan Knoll, Labrador Sea

Abstract

Deep-sea mounds can have a variety of origins and may provide hard-substrate features in depths that are normally dominated by mud. Orphan Knoll, a 2 km high bedrock horst off northeast Newfoundland, hosts more than 200 mounds, or mound complexes, of unknown composition, in water depths of 1720-2500 m. Most mounds are 10-600 m high, with average mound height 187 m, and 1-3 km wide. The study objective was to characterize the size, shape, orientation, and composition of the enigmatic Orphan Knoll mounds, in order to determine their age and origin. Archival ship-based side-scan sonar, multibeam sonar, airgun, high-resolution sparker and 3.5 kHz acoustic sub-bottom profiling, and newly acquired ship-based multibeam sonar, video transects by remotely operated vehicle (ROV), rock samples, and near-bottom multibeam sonar data were analyzed. Four mounds were studied during two ROV dives. Archival sidescan sonar data show > 200 mounds. Sparker profiles show that the mound crests are covered by condensed stratified Quaternary sediment and airgun seismic data show faults reaching near the seafloor. New multibeam sonar data show mounds are dominantly conical to elliptical in shape, but without preferred orientation or alignment. Remotely operated vehicle (ROV) transects and near-bottom multibeam showed that three mounds were rounded and symmetrically arranged, while a fourth was more asymmetrical, with steep faces on the southwestern and southeastern flanks, where finely bedded to massive sedimentary bedrock outcropped dipping 15-45 • SW. Rock samples from the mounds include Eocene calcareous ooze and mid-Miocene bedded pelagic limestone. Thick ferromanganese crusts were found on many surfaces, obscuring possible outcrops from physical sampling. Polymetallic nodules were found on the slope of one mound. Ice-rafted detritus, including igneous and metamorphic rocks and Paleozoic limestone and dolostone, was common in the sediments immediately surrounding the mounds. Quaternary sub-fossil solitary scleractinian corals accumulated over a span of at least 0.18 Ma at the base of one mound. The presence of uplifted condensed Eocene-Miocene rocks on the mounds and faulting in seismic profiles suggest uplift during Frontiers in Marine Science | www.frontiersin.org 1 January 2020 | Volume 6 | Article 744 Meredyk et al. Orphan Knoll Mounds reactivation of old rift-related faults during the Neogene, with seabed mass wasting creating residual mounds, which were then draped by Quaternary proglacial muds. Sculpting of hemipelagic Quaternary sediment by bottom currents probably contributed to mound morphology.

Authigenic and Detrital Carbonate Nd Isotope Records Reflect Pulses of Detrital Material Input to the Labrador Sea During the Heinrich Stadials

“…The ɛ Nd signatures of the residual silicate digests of the uppermost samples of the WLS and BIS cores are 8–9 ɛ Nd units more radiogenic than those of the residual silicate digests of the surface sediment samples from the western Labrador Slope (sample 3, ɛ Nd ∼ −27.2, Figure 1, Table ), whereas for the Orphan Knoll core the residual silicate ɛ Nd signature of the uppermost sample of ∼ −20.9 is close to the range of surface sediment samples analyzed in this study (samples 2,4,5,6, Figure 1; ɛ Nd between −22 and −26, Table ). The core‐top leachate ɛ Nd signatures off the Orphan Knoll and off Iceland show better agreement with the modern seawater data from nearby stations (Lacan & Jeandel, 2005; Lambelet et al., 2016; Figure S1 in the Supporting Information ), whereas more variability and offsets between bottom seawater and authigenic ɛ Nd signatures are observed along the western Labrador shelf that could reflect the complex bathymetry of the region complicated by presence of numerous mounds in the area that can affect bottom current movement (Meredyk et al., 2020; Figure S1 in the Supporting Information ).…”

Large-scale bedrock outcrop mapping on the NE Atlantic Irish continental margin

“…Ireland has extensive nearvertical outcrop along its continental margin that can be sampled using cost-effective ROV-sidewall drilling (Lim et al, 2018). However, very few studies target direct bedrock observations in very deep settings (Brown et al, 2019;Trotter et al, 2019;Backus et al, 2020;Meredyk et al, 2020) on a regional scale, especially in the deep-sea territory of offshore Ireland. In this way, the nearsurface geology of the southern ICM remains poorly known.…”

Increasing Knowledge of Biodiversity on the Orphan Seamount: A New Species of Tedania (Tedaniopsis) Dendy, 1924

“…Orphan Knoll is an isolated, drowned continental fragment 550 km northeast of Newfoundland in the Labrador Sea (NW Atlantic) (Meredyk et al, 2020). The top of Orphan Knoll stands at 1,800-2,000 m and is marked by a series of protruding mounds at depths of between 1,800 and 2,300 m. The Orphan Seamount is located 9 km northeast of the southern-most extension of Orphan Knoll and is a volcanic seamount (Meredyk, 2017).…”

“…The transect took place between depths of 1,862 and 3,004 m, with an average of 2,455 m and a distance traveled of 11,515 m (Wudrick et al, 2020). This new species of the genus Tedania Gray, 1867, was collected close to 3,000 m in depth, inside the NAFO closed area and CBD EBSA, where the substrate were rocks of volcanic origin (Meredyk et al, 2020).…”