Abstract
Macrofauna is known to inhabit the top few 10s cm of marine sediments, with rare burrows up to two metres below the seabed. Here, we provide evidence from deep-water Permian strata for a previously unrecognised habitat up to at least 8 metres below the sediment-water interface. Infaunal organisms exploited networks of forcibly injected sand below the seabed, forming living traces and reworking sediment. This is the first record that shows sediment injections are responsible for hosting macrofaunal life metres below the contemporaneous seabed. In addition, given the widespread occurrence of thick sandy successions that accumulate in deep-water settings, macrofauna living in the deep biosphere are likely much more prevalent than considered previously. These findings should influence future sampling strategies to better constrain the depth range of infaunal animals living in modern deep-sea sands. One Sentence Summary: The living depth of infaunal macrofauna is shown to reach at least 8 metres in new habitats associated with sand injections.
Highlights
Deep-sea infauna is one of the most elusive branches of life on Earth; little is known about modern deep seabed environments, and less about the ancient
The structures on injectite margins are interpreted as trace fossils, and not grooves or markings formed through the injection process because they include the branching structure of Thalassinoides and gently sinuous burrows of Planolites (Fig. 3A)
Our findings have several biological and geological implications, i) unusually, we can quantify a minimum depth below the seabed that organisms inhabited in ancient sediments, ii) show that the deepest organisms may be present in sandy sediments, rather than the clays and silts typically targeted in modern seabed investigations, iii) show that less organics are preserved due to carbon consumption during metabolic activity, which changes the sediment fabric at depth, with grains being processed and sorted into burrow structures, and iv) most importantly, we have shown that macrofaunal life survives for periods living at depths of up to 8 m below the seabed, giving an entirely new limit to the macrofaunal biosphere
Summary
Three separate Permian outcrop sites from the SW Karoo Basin, South Africa (Fig. 1) exhibit sand injectites sourced from deep-marine turbidite sands in the Fort Brown Formation. Bioturbation is documented throughout this formation[10] and ichnological assemblages include Thalassinoides and Planolites[11], commonly observed as hypichnia on sandstone bed bases (Fig. 2). Sandstone beds throughout the Fort Brown Formation are observed on the margins of clastic injectites (Fig. 3), down to these lower limits. The structures on injectite margins are interpreted as trace fossils, and not grooves or markings formed through the injection process because they include the branching structure of Thalassinoides and gently sinuous burrows of Planolites (Fig. 3A). The organisms forming Planolites and Thalassinoides have been believed to have lived mainly in the top 20 cm of sediment, rarely reaching maximum depths of 1.5 m3,15.
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