Micro-trace fossils reveal pervasive reworking of Pliocene sapropels by low-oxygen-adapted benthic meiofauna

Abstract

Animal burrowers leave an indelible signature on the sedimentary record in most marine environments, with the seeming exception of low-oxygen environments. In modern sedimentary settings, however, sub-millimetre-sized benthic animals (meiofauna) are adapted to low oxygen and even sulfidic conditions. Almost nothing is known about their impact on ancient marine sediments because they leave few recognizable traces. Here we show, in classic Pliocene-aged anoxic facies from the Mediterranean, the first reported trace fossil evidence of meiofaunal activity and its relation to changing oxygenation. A novel approach utilizing electron imaging of ion-polished samples shows that meiofauna pervasively reworked sediment under oxygen-depleted conditions that excluded macrofauna, fragmenting organic laminae and emplacing 15-to 70-mm-diameter faecal pellets without macroscopically influencing the fabric. The extent of reworking raises the question: how pervasively altered are other sediments presently assumed to lack animal influence and how far into the geological record does this influence extend?

Sediment microfabric records mass sedimentation of colonial cyanobacteria and extensive syndepositional metazoan reworking in Pliocene sapropels

“…Microfacies 4A is characterized by fragmentation and deformation of diffuse and discrete organic‐rich laminae, with the degree of reworking increasing upsection. Deformed OM‐rich laminae fragments are accompanied by two types of organic‐rich, faecal pellets (Figure a–d), corresponding to the type A and type B pellets of Löhr and Kennedy (). Type A pellets are 15–35 μm (short axis) and 30–70 μm (long axis intersected) in size, round to oval in shape and occur throughout the bioturbated interval.…”

“…Type B pellets are larger, 60–100 μm (short axis) and 100–300 μm (long axis), and are present mainly within the top 1 cm of the bioturbated sapropels. Löhr and Kennedy () interpreted both morphotypes to be of benthic meiofaunal origin, representing reworking of sapropels under low‐oxygen conditions prohibitive to macrofaunal burrowing organisms. Both consistently co‐occur with evidence of in situ sediment disruption including OM laminae fragmentation and sediment homogenization (Figure a,b).…”

“…Macrofaunal trace fossils such as Chondrites , recording postdepositional benthic reworking, are present in the upper portions of many sapropels (Löhr & Kennedy, ; Löwemark et al., ; Shipboard Scientific Party, ), but comprehensive, syndepositional macrofaunal burrowing and emplacement of Zoophycus traces such as that reported for two Pliocene sapropels at Site 969, is unusual (Shipboard Scientific Party, ). Although benthic foraminiferal repopulation events indicative of episodic deep water ventilation are present in sapropels spanning the Pliocene to the Holocene (Casford et al., ; Jilbert, Reichart, Mason, & de Lange, ; Rohling, de Stigter, Vergnaud‐Grazzini, & Zaalberg, ; Rohling et al., ; Schmiedl et al., ), and low‐level benthic foraminiferal fauna are known to persist through the entire thickness of a smaller number of sapropels (Casford et al., ; Jorissen, ; Rohling, Jorissen, & Stigter, ), bottom water conditions are generally considered too hostile to support a metazoan benthos.…”

Ichnological evidence for meiofaunal bilaterians from the terminal Ediacaran and earliest Cambrian of Brazil

“…Although meiofaunal burrows (sometimes described as burrow mottling or cryptobioturbation) have occasionally been reported from Cambrian to Recent sediments 26 , they are rarely subjected to detailed study. Body fossil discoveries also reveal organisms inhabiting meiofaunal niches within Early Cambrian communities, highlighting the potential for their preservation within particular taphonomic windows 28,29 .…”

“…Modern meiofaunal communities include animals, foraminifera and some ciliates, and contribute significantly to sediment bioturbation and bioirrigation 26,27 . The meiofauna can be divided into permanent members (that is, animals with organisms of a small size adapted and restricted to the meiofaunal, interstitial realm) and temporary meiofauna (for example, the larvae of macrobiota) 25 .…”