Journal Article

The Enigmatic Pockmarks of the Sandy Southeastern North Sea

Hoffmann, Jasper J. L., Geersen, Jacob, Sander, Lasse, Schmidt, Christopher, Schneider von Deimling, Jens, Karstens, Jens, Schmidt, Mark, Müller, Thomas Harald, Spiegel, Timo, Böttner, Christoph, Unverricht, Daniel, Andresen, Katrine Juul

2024

Overview

Natural seafloor depressions, known as pockmarks, are common subaqueous geomorphological features found from the deep ocean trenches to shallow lakes. Pockmarks can form rapidly or over millions of years and have a large variety of shapes created and maintained by a large variety of mechanisms. In the sandy sediments of the southeastern North Sea, abundant shallow pockmarks are ubiquitous and occur at shallow water depths (<50 m). Their formation has previously been linked to methane seepage from the seafloor. Here, we characterize over 50,000 pockmarks based on their morphology, geochemical signature, and the subsurface pre‐conditions using a new integrated geoscientific data set, combining geophysical and sedimentological data with geochemical porewater and oceanographic analysis. We test whether the methane seepage is indeed responsible for pockmark formation. However, our data suggest that neither the seepage of light hydrocarbons nor groundwater is driving pockmark formation. Because of this lack of evidence for fluid seepage, we favor the previously suggested biotic formation but also discuss positive feedback mechanisms in ocean bottom currents as a formation process. Based on a comparison of pockmarks to the central and southeastern North Sea, we find that local lithology significantly affects pockmark morphology. Muddy lithologies favor the formation of larger, long‐lived structures, while sandy lithologies lead to short‐lived, small‐scale structures that are large in area but with shallow incision depth. We conclude that pockmarks in sandy environments might have been overlooked globally due to their shallow incision depth and recommend reevaluating the role of hydrocarbon ebullition in pockmark formation. Plain Language Summary Pockmarks are natural depressions on the seafloor, found everywhere from deep oceans to shallow lakes. These underwater features can form quickly or over millions of years, sometimes reaching impressive sizes—tens of meters deep and kilometers wide. Pockmarks are created by different processes depending on where they are found, but they are often linked to the release of gases such as methane from the seafloor. In the southeastern North Sea, thousands of shallow pockmarks have been discovered in sandy areas less than 50 m deep. Initially thought to be caused by methane gas leaks, our research shows this is unlikely. We studied over 50,000 pockmarks using geological, chemical, and oceanographic data and found little evidence that methane or groundwater seepage is responsible. Instead, these pockmarks are more likely created by either marine animals such as harbor porpoises or turbulent ocean currents. We also found that the grainsize of the sediment greatly affects pockmark morphology and lifespan. In muddy areas, pockmarks are larger and longer‐lasting, whereas in sandy areas, they are shallower and shorter‐lived. Our findings suggest that many pockmarks in sandy environments may have been overlooked due to their shallow depth. We recommend reconsidering the role of methane in pockmark formation. Key Points Pockmarks in the German Bight form in sandy environments and are likely not related to fluid venting Pockmarks in sandy sediments are shallow in depth, large in area and short‐lived structures compared to muddy host sediments Despite the widespread occurrence of pockmarks in the southeastern North Sea, the processes leading to their formation remain unclear

• Publisher: John Wiley & Sons, Inc,Wiley
• Subject: Bottom currents / fluid flow / Gas seepage / Geochemistry / Geologic depressions / geomorphology / Groundwater / Harbors / Hydrocarbons / interdisciplinary / Lakes / Lithology / Marine mammals / Marine organisms / Methane / Morphology / North Sea / Ocean currents / Ocean floor / Oceanic trenches / Oceanographic data / Oceans / pockmarks / Pore water / Positive feedback / Sediment / Seepage / Shallow water / Water depth