Bioturbation in the hadal zone

Abstract

The hadal zone, >6 km deep, remains one of the least understood ecosystems on Earth. We address bioturbational structures in sediment cores from depths exceeding 7.5 km, collected during the IODP Expedition 386 in the Japan Trench. Micro-CT imaging on 20 core sections allowed to identify biogenic sedimentary structures (incipient trace fossils) and their colonization successions within gravity flow deposits. Their frequency, and consequent changes in substrate consistency, oxygenation and organic matter delivery and remineralization controlled the endobenthic colonization. The gravity-flow beds show recurring bioturbation successions: The initial colonization is characterized by deposit-feeding structures such as Phycosiphon, Nereites and Artichnus generating typically 20 cm thick intensively bioturbated fabrics. The final colonization stage comprises slender spiral, lobate and deeply penetrating straight and ramifying burrow systems such as Gyrolithes, Pilichnus and Trichichnus, interpreted to include burrows of microbe farming and chemosymbiotic invertebrates. The main factor precluding colonization is soupy substrate. Organic matter degradation and post-event upward expansion of the anoxic zone drive the change from deposit feeding to microbe-dependent feeding strategies.

Fig. 1. Map of the Japan Trench (modified after Kioka et al. 2019).

Red dots indicate site locations of the studied cores. Black bold and dashed lines mark the Nakaminato and Ogawara canyon in the south and north, respectively.

Fig. 2. Main sedimentological characteristics of the studied cores.

A Stacked, sharp-based and base-graded event beds comprising recurring facies successions: Base of the successions is formed by ripple cross-laminated silt (F3), which is followed by laminated clay and silt (F2). These deposits are, in turn, overlain by structureless mud (F2C) and/or bioturbated mud (F1B). Red arrow – low-angle ripple cross-lamination. See text for explanation. Core section M0081D-1H-6WR2, 0–30 cm, Southern Japan Trench expanded section. B Close up of the base of a gravity-flow bed, showing ripple cross-laminated silt (F3) that grades upward into laminated clay and silt (F2). The lamination starts with subtle, lateral thickness variation with laminae terminations, followed by parallel lamination and transition to structureless mud (F2C). C Slump-folding and loading structures in clay-silt heteroliths. The uppermost ripple cross-lamination grades laterally into laminated mud (blue arrow). Soft-sediment deformation is restricted to a specific bed interval pointing to syn-sedimentary origin and high initial water content of the sediment. Core section M0082C-1P-1WR6, 63–72 cm, Southern Japan Trench condensed section. D Laminated to cross laminated mud with foresets (red arrow) and down-lapping laminae terminations (yellow arrow). Core section M0082C-1P-1WR6, 74–80 cm, Southern Japan Trench condensed section. E High-contrast 3D-view to pseudo-nodular silt/sand, sunken into structureless mud. Core section M0083F-1H-10WR16, 0–21 cm, Central Japan Trench expanded section. Note that the core margins have bent downwards due to coring process.

Fig. 3. Traces assigned to early post-event suites. White vertical lines mark the margin between image surfaces oriented in right angle.

A Fugichnia (fu) escape burrow extending upwards from clay-silt lamination (F2) to structureless mud (F1C). Core section M0085D-1H-3WR27, 92–94 cm, Northern Japan Trench condensed section. B, C Oblique side- and frontal-view of Rhizocorallium, respectively. Yellow arrows indicate spreite. Core section M0081D-1H-6WR2, 4.5–6.5 cm, Southern Japan Trench expanded section. D–I Various expressions of Rosselichnid traces. Red arrows point to potential Artichnus, whereas blue arrows indicate sub-vertical elements. In (D, E), Rosselichnid traces cross-cut a fabric dominated by Phycosiphon (Ph) and Nereites (Ne), respectively. Core sections: (D) M0089D-1H-5WR36, 29.5–40.5 cm, Central Japan Trench condensed section; (E) M0085D-1H-3WR27, 95–100 cm, Northern Japan Trench condensed section; (F, G) M0082D-1H-2WR7, 80–92 cm and 90–100 cm, respectively, Southern Japan Trench condensed section; H and I) M0081F-1H-7WR4, 96–101 cm, Southern Japan Trench expanded section. Note that the core margins have bent downwards due to coring process.

Fig. 4. High-contrast 3D-views to traces assigned to the equilibrium suite.

A, B Helicoidal burrows referred to as cf. Gyrolithes locraensis. Core section M0082D-1H-2WR7, 75.5–82.5 cm, Southern Japan Trench condensed section. C Long and narrow threads of Trichichnus isp. Core section M0089D-1H-5WR36, 34–40 cm, Central Japan Trench condensed section. D, E Irregular and variably oriented ramifying burrow system (Pilichnus), with local helicoidal segments (red arrow). Upwards, towards the colonization surface, the Chondrites-like traces can be associated with few mm wide vertical shafts (yellow arrow). Core section M0089D-1H-5WR35, 15–24 cm, Central Japan Trench condensed section. F A close up to the colonization surface illustrating vertical shafts related to Pilichnus. Red arrow points to a Gyrolithes fragment. The surface is affected by erosion generated by the subsequent gravity flow event. M0083F-1H-10WR16, 0–21 cm, Central Japan Trench expanded section. G, H A Zoophycos-like narrow lobate burrow. Core section M0082D-1H-3WR8, 18–24 cm, Southern Japan Trench condensed section. Note that the core margins have bent downwards due to coring process.

Fig. 5. Results of statistical analysis.

A, B Plots of non-metric multidimensional scaling results.The points are numbered according to coring sites. The closer the points plot to each other, the more similar they are. Ellipses indicate 95 % confidence intervals of the standard deviation. C, D Plots of sedimentary total C contents and Artichnus diameter in Southern, Central and Northern Japan Trench. Red vertical bars indicate interquartile ranges. Median value points are connected by a blue line. The numbers are p-values of two-sided Wilcoxon rank sum tests with Bonferroni correction for multiple comparisons. E Spearman rank correlation matrix of bioturbation properties, sediment grain size median and total C, N and S contents. F Plot of sediment total C content versus Artichnus diameter with Spearman rank correlation coefficients and a linear regression line. The shaded band indicates the 95% confidence interval. P-values < 0.05 are considered statistically significant. n – number of observations. Source data for Fig. 5 are available as a Source Data file.

Fig. 6. A sedimentological-ichnological model for event bed colonization in the Japan Trench.

See text for further explanation. O–Oxygen.