Observations of Pockmark Flow Structure in Belfast Bay, Maine, Part 1: Current-Induced Mixing

Geo-Mar Lett DOI 10.1007/s00367-016-0472-4 ORIGINAL Observations of pockmark flow structure in Belfast Bay, Maine, Part 1: current-induced mixing Christina L. Fandel1,2 & Thomas C. Lippmann1 & James D. Irish1 & Laura L. Brothers3 Received: 5 April 2016 /Accepted: 25 September 2016 # Springer-Verlag Berlin Heidelberg 2016 Abstract Field observations of current profiles and tempera- and suggest that form drag may possibly be influencing the ture, salinity, and density structure were used to examine ver- local flow regime. While resource limitations prevented ob- tical mixing within two pockmarks in Belfast Bay, Maine. The servation of the current structure and water properties at a first is located in 21 m water depth (sea level to rim), nearly control site, the acquired data suggest that active mixing and circular in shape with a 45 m rim diameter and 12 m rim-to- overturning within the sampled pockmarks occur under typi- bottom relief. The second is located in 25 m water depth, more cal benign conditions, and that current flows are influenced by elongated in shape with an approximately 80 m (36 m) major upstream bathymetric irregularities induced by distant (minor) axis length at the rim, and 17 m relief. Hourly aver- pockmarks. aged current profiles were acquired from bottom-mounted acoustic Doppler current profilers deployed on the rim and center of each pockmark over successive 42 h periods in Introduction July 2011. Conductivity–temperature–depth casts at the rim and center of each pockmark show warmer, fresher water in Pockmarks are roughly conical depressions in the seafloor that the upper water column, evidence of both active and fossil are distributed in a variety of global geologic environments thermocline structure 5–8 m above the rim, and well-mixed ranging from shallow coastal regions (e.g., Wildish et al. water below the rim to the bottom. Vertical velocities show 2008;Brothersetal.2011a, b), to deep offshore settings up- and down-welling events that extend into the depths of (e.g., Hovland and Judd 1988; Hovland and Svensen 2006). each pockmark. An observed temperature change at both the These crater-like features range in size from less than 1 m to rim and center occurs coincident with an overturning event over a kilometer in diameter. In Belfast Bay, located in the below the rim, and suggests active mixing of the water column northwestern quadrant of Penobscot Bay in central Maine, into the depths of each pockmark. Vertical profiles of horizon- USA, there exists an extensive field of over 1,700 pockmarks tal velocities show depth variation at both the center and rim within a 25 km2 area (Fig. 1; Scanlon and Knebel 1989; consistent with turbulent logarithmic current boundary layers, Andrews et al. 2010). Methane release from Holocene estuarine mud is considered to be the primary mechanism of pockmark formation in Belfast Bay, yet the relative degassing his- * Thomas C. Lippmann tory and maintenance mechanisms of the pockmarks remain [email protected] unresolved (Kelley et al. 1994), similar to other areas world- wide (e.g., Scanlon and Knebel 1989; Christodoulou et al. 1 University of New Hampshire, Center for Coastal and Ocean 2003). Although timing and activity of pockmark formation Mapping, 24 Colovos Rd., Durham, NH 03824, USA is highly uncertain, many pockmarks are believed to have 2 Present address: Hydrographic Surveys Division, Office of Coast been inactive for long periodsoftime(perhapsyearsor Survey, National Oceanic and Atmospheric Administration, 1315 more; e.g., Kelley et al. 1994). Thus, the question remains as East-West Hwy, Silver Spring, MD 20910, USA to why inactive pockmarks do not—over time—fill in with 3 U.S. Geological Survey, 384 Woods Hole Rd., Woods sediments, and for non-actively venting pockmark fields, it is Hole, MA 02543, USA not clear what the mechanisms are for long-term maintenance. Geo-Mar Lett Fig. 1 Overview map of Belfast Bay, Maine, with the approximate location of each sampled pockmark (green box). Weather stations 1 and 2 (blue triangles) are located approximately 2.4 km east of Belfast and Searsport, Maine, respectively Although much research has addressed pockmark forma- the North Sea show stratigraphic sediment layering consistent tion, specifically in Belfast Bay (e.g., Scanlon and Knebel with elongation of initially circular pockmarks, and subse- 1989;Kelleyetal.1994;Rogersetal.2006;Brothersetal. quent coalescing of neighboring pockmarks that form km long 2012), few studies have focused on direct observations of (multiple) furrows suggesting that post-formation processes circulation, mixing, and sediment characteristics within modify pockmark fields through sediment transport mecha- pockmarks. One notable exception is a recent field study by nisms (Kilhams et al. 2011). However, more field studies that Pau and Hammer (2013) who include observations of sedi- observe flows and mixing within the pockmarks are needed to ment grain size and geochemical composition, as well as improve understanding of sediment transport mechanisms for acoustic backscatter and flow above pockmarks in Oslofjord, post-formation pockmarks, better constrain models for age Norway. Their work suggests that deposition in pockmarks approximation, and guide models for improved estimation of may be determined by the relative importance of bedload mixing and nutrient distribution within individual pockmarks and suspended sediment transport patterns, or by biological (e.g., Gay et al. 2005; Hovland et al. 2005; Wildish et al. 2008; activity (i.e., swimming fish) that induce diurnal circulation Ritt et al. 2011). patterns over the sampled pockmark (Pau and Hammer 2013). In Part 1 of this study (this paper), field observations ob- Although lack of horizontal and vertical flow measurements tained within the depths (near the bottom) and the rim of two within the depths of the pockmarks prevent direct observation pockmarks in Belfast Bay, Maine, are presented and of the deep circulation or mixing of fluid into and out of the discussed. Both pockmarks show evidence of active mixing depressions, their work suggests that pockmarks in Oslofjord of water above and below the rim, and a vertical flow structure may be maintained by modern conditions. Seismic records in with boundary layers induced by both internal pockmark Geo-Mar Lett circulation and external flows influenced by upstream bathymetric irregularities. In Part 2 (Fandel et al. 2016a), circulation patterns and horizontal shear layer growth within the pockmarks are shown to be consistent with open cavity flows and previously modeled flow patterns (Brothers et al. 2011a;Pau et al. 2014). Finally, in Part 3 (Fandel et al. 2016b), observed horizontal and vertical currents are examined in conjunction with the cohesive sediment characteristics to characterize periods of sediment resuspension and deposition within and around the pockmarks. Materials and methods Field observations of the temperature, salinity, density, and current structure were obtained over the rim and center of two pockmarks with conductivity, temperature, and depth (CTD) casts and bottom-mounted, upward-looking acoustic Doppler current profilers (ADCPs). Candidate pockmarks were selected from a 5 by 5 m resolution bathymetric map of Belfast Bay produced from interferometric sonar data acquired by the U.S. Geological Survey in cooperation with the University of Maine in 2006 and 2008 (Brothers et al. 2011a). Characterized as a shallow estuarine environment (10–50 m water depths), Belfast Bay receives fresh-water input from the lower flow Passagassawakeag River to the northwest and the higher flow Penobscot River to the northeast (US Geological Survey 2010). Pockmark morphology transitions from a nearly circular shape in the shallower, northern region to a more elongated shape in the deeper, southern region, where the bay Fig. 2 Color-filled contour plots show bathymetric data collected over significantly constricts. Two pockmarks were selected based the northern (top panel) and southern (bottom panel) pockmark with x on proximity to other pockmarks, depth restrictions of the and y axes showing the estimated distance from the center of each instrumentation, and characteristic morphology (Fig. 2). A pockmark. White dots Approximate location of each current meter mount along the rim and center of each pockmark, with associated geographic right-handed coordinate system is adopted with uncertainty denoted by the size of each dot. Elevations are referenced to vertical datum at mean sea level and z-coordinate positive mean sea level upward. Water depths are reported as positive values. The first pockmark selected was nearly spherical with a 45 m diameter at the rim, and located in 21 m water 209.9–211.6). Field observations were not acquired at a depthwith12mrelief(depression) from the rim to the control site due to resource limitations. bottom. The second selected pockmark is located to the Hourly averaged wind speed, direction, and gust speed from south in 25 m water depth, and is more elongated with meteorological stations located onshore near the townships of 80 m major axis and 36 m minor axis lengths, and 17 Belfast and Searsport, Maine (Fig. 1) were collected and dis- m relief below the rim. The northern pockmark is locat- tributed by Weather Underground, Inc. (2011). Anemometers ed near a complex convergence zone of tidal currents were located about 30 m and 15 m above mean sea level at the flowing around Islesboro Island, whereas the southern Belfast and Searsport

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