Recovery of Paleodictyon patterns after simulated mining activity on Pacific nodule fields

Abstract

Since the late 1980s, various experiments have been conducted in polymetallic nodule fields of the Pacific Ocean to assess the potential environmental impacts of future mining, specifically in two areas: the Peru Basin and the Clarion-Clipperton Fracture Zone (CCZ). Two expeditions, SO242/2 in 2015 (Peru Basin) and SO268/1 + 2 in 2019 (CCZ), deployed a towed camera system to collect imagery from both areas. These expeditions aimed to assess recovery of fauna in the short (few weeks) and long term (several years) following physical seafloor disturbance actions designed to mimic potential mining, by ploughs, dredges and epibenthic sleds. Within the collected image data, several strikingly hexagonal hole patterns were observed and identified as Paleodictyon nodosum, and an irregular form of Paleodictyon traces, both on undisturbed and disturbed areas of seafloor. Recent forms occur abundantly in various deep-sea regions, but their origin, and how they represent the mode of life of the forming organism, remains unknown. In this study, the imaged occurrences of Paleodictyon traces on disturbed seafloor sheds light on the lifecycle of the forming organism, demonstrating that they can recolonize disturbed habitat and produce the trace network in a few weeks. Nevertheless, the density of these patterns on disturbed substrates was lower than observed on undisturbed substrates in both nodule regions. We therefore hypothesize that, along with other benthic deep-sea fauna, these structures and the forming organism are impacted by physical seafloor disturbance, and even 26 years after disturbance, densities on disturbed sediments have not recovered to undisturbed levels.

Northernmost (Subarctic) and deepest record of Paleodictyon: paleoecological and biological implications

“…The Aleutian examples are considerably smaller than the previously published examples, except for some specimens from the DEA (Fig. 2 e in Boehringer et al 8 ).…”

“…The density of Paleodictyon in the study area was considerably lower than the maximum densities (45 indiv./m 2 )found at the MAR 5 or along the Indian Ridge (9.7 indiv./m 2 ), where patchy distributions were observed 9 . Also, the densities in the Aleutian area were three orders of magnitude lower than the average values reported for the CCZ (0.3 indiv./m 2 in Durden et al 7 ; and 0.2 indiv./m 2 in Boehringer et al 8 ) but higher than those reported in the DISCOL Experimental Area (DEA) in the Southeastern Pacific (0.0033 indiv./m 2 in Boehringer et al 8 ). Mesh sizes (usually referred as diameter) reported in the literature span from 1.8 to 7.5 cm, with the average size always being more than 2.5 cm 5 – 9 .…”

“…In fact, the majority of described specimens have been assigned to the relatively large ichnospecies Paleodictyon nodosum , which reaches a size of up to 7.5 cm 5 , 7 , 9 . The recent study of Boehringer et al 8 is the only one to distinguish two types of modern Paleodictyon and analyze them separately. The absence of a greater diversity of modern Paleodictyon morphotypes may be related to the morphometrics used.…”

“…Also, a consistent environmental requirement for Paleodictyon appears to be the presence of soft, fine-grained sediments 5 , 9 , 14 . Traces have been observed even when fine sediments are associated with hard substrates, for example, polymetallic nodule fields in the equatorial Pacific where nodules have been observed to interrupt Paleodictyon patterns 7 , 8 , or patchy sediment overlying a basalt substrate in the central Indian Ocean 9 . The sediments are usually calcareous Globigerina ooze, but the Aleutian traces occur on siliceous oozes with radiolarians and diatoms (A.J.…”

Impact of returning scientific cruises and prolonged on-site presence on litter abundance at the deep-sea nodule fields in the Peru Basin

“…26 years after the disturbance experiments the plough tracks were still very prominent and the impact on the fauna significant (e.g. Boetius and Haeckel, 2018, Stratmann et al, 2018, Simon-Lledó et al, 2019, Gausepohl et al, 2020, Vonnahme et al, 2020, Boehringer et al, 2021.…”