More than a century of bathymetric observations and present-day shallow sediment characterization in Belfast Bay, Maine, USA: implications for pockmark field longevity

Abstract

Mechanisms and timescales responsible for pockmark formation and maintenance remain uncertain, especially in areas lacking extensive thermogenic fluid deposits (e.g., previously glaciated estuaries). This study characterizes seafloor activity in the Belfast Bay, Maine nearshore pockmark field using (1) three swath bathymetry datasets collected between 1999 and 2008, complemented by analyses of shallow box-core samples for radionuclide activity and undrained shear strength, and (2) historical bathymetric data (report and smooth sheets from 1872, 1947, 1948). In addition, because repeat swath bathymetry surveys are an emerging data source, we present a selected literature review of recent studies using such datasets for seafloor change analysis. This study is the first to apply the method to a pockmark field, and characterizes macro-scale (>5 m) evolution of tens of square kilometers of highly irregular seafloor. Presence/absence analysis yielded no change in pockmark frequency or distribution over a 9-year period (1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008). In that time pockmarks did not detectably enlarge, truncate, elongate, or combine. Historical data indicate that pockmark chains already existed in the 19th century. Despite the lack of macroscopic changes in the field, near-bed undrained shear-strength values of less than 7 kPa and scattered downcore 137 Cs signatures indicate a highly disturbed setting. Integrating these findings with independent geophysical and geochemical observations made in the pockmark field, it can be concluded that (1) large-scale sediment resuspension and dispersion related to pockmark formation and failure do not occur frequently within this field, and (2) pockmarks can persevere in a dynamic estuarine setting that exhibits minimal modern fluid venting. Although pockmarks are conventionally thought to be long-lived features maintained by a combination of fluid venting and minimal sediment accumulation, this suggests that other mechanisms may be equally active in maintaining such irregular seafloor morphology. One such mechanism could be upwelling within pockmarks induced by near-bed currents.

Shallow stratigraphic control on pockmark distribution in north temperate estuaries

“…Inconsistent evidence for venting (Kelley et al, 1994;Ussler et al, 2003), a dearth of methanotropes (Wildish et al, 2008), and nominal seafloor change (Brothers et al, 2011a) suggests these pockmark fields currently experience minimal fluid-escape activity. Geochemical sampling (Ussler et al, 2003) suggest that the region's methane is microbial in origin, although the stratigraphic source of the gas remains undetermined.…”

Linking Geology and Microbiology: Inactive Pockmarks Affect Sediment Microbial Community Structure

“…Areas with many pockmarks are often stable in the number of pockmarks since subsurface gas or fluid flow usually tends to follow the existing venting channels instead of creating novel ones [5]. Finally, surveys of the seabed indicate that inactive pockmarks outnumber the active pockmarks [2], [7].…”

Methane and minor oil macro-seep systems — Their complexity and environmental significance

“…Pockmarks are generally associated with any kind of fluid flow, where the fluids (gas, and/or liquids) originate from any depth in the subsurface (Judd and Hovland, 2007). Even though the first to discover and name pockmarks, King and MacLean (1970) suggested them to be solely related to hydrocarbon-prone areas, the occurrence of pockmarks in areas underlain by metamorphic basement rocks (Pinet et al, 2010;Brothers et al, 2011) and hydrothermal activity, clearly demonstrates that thermogenic fluids and derivatives (biogenic methane) are not the only fluids responsible for these morphological features (Kelley et al, 1994). Thus, the fluids responsible for pockmarks may be any fluid, ranging from groundwater to deeply sourced CO 2 , CH 4 , or locally sourced fluids of biogenic origin associated with the degradation of recently buried, organic-rich material.…”

“…The same can also be said about some natural offshore oil seeps. Lately, the seepage of methane and heavier hydrocarbons through the seafloor and through lake beds have achieved increasing scientific focus for various reasons (Niemann et al, 2005;Whelan et al, 2005;Judd and Hovland, 2007;Jensen et al, 2008aJensen et al, , 2008bWegener et al, 2008;Hovland et al, 2010;Plaza-Faverola et al, 2010;Valentine et al, 2010;Wessels et al, 2010;Brothers et al, 2011;Dondurur et al, 2011;Hammer et al, 2011;Krylova et al, 2011;Pape et al, 2011;Redmond and Valentine, 2011;Reitz et al, 2011;Solomon et al, 2011;Batang et al, 2012). One of the reasons is that scientists and the general public are becoming more concerned about the carbon cyclus, in general, and the radiative (greenhouse) gases in particular (Hovland et al, 1993;Kennett et al, 2000;Mienert et al, 2005;Etiope et al, 2008;Westbrook et al, 2009;Canet et al, 2010;Brett et al, 2011).…”