Deep-Sea Ichnofabric Responses to Bottom-Current Velocity Oscillations at Sub-Millennial Scales

Trace fossils serve as crucial proxies for interpreting paleoenvironmental changes, yet their potential remains constrained by sedimentary continuity and vertical resolution, leaving sub-millennial-scale benthic behavioral responses to environmental variations poorly understood. This study examines an LGM interval (18.86–20.19 ka) of deep-water contouritic records with exceptionally high sedimentation rates (>80 cm/kyr). Using CT scanning, grain size analysis, and XRF data, we elucidate how ichnofabrics respond to periodic fluctuations in bottom-current velocity. The trace-fossil assemblages mainly comprise shallow-tier burrows (e.g., Nereites, Scolicia), medium-tier burrows (e.g., Techildnus, Thalassinoides), and deep-tier burrows (e.g., Chondrites). Cross-cutting relationships of trace fossils reveal two colonization sequence types: (1) Normal=deeper-tier burrows penetrate shallower-tier ones; (2) Abnormal=shallower-tier burrows penetrate deeper-tier ones, indicating erosions or hiatuses between colonization stages. Abnormal relationships of Scolicia penetrate Techildnus and Chondrites, respectively, are recognized. The deep-tiering ichnofabric T3 comprises shallow-, medium-, and deep-tier burrows, averaging 10.53 cm in thickness; middle-tiering ichnofabric T2 comprises shallow- and medium-tier burrows, averaging 7.61 cm; shallow-tiering ichnofabric T1 comprises only shallow-tier burrows, averaging 2.34 cm. T3 requires the longest burrowing time, and the depth position of its top boundary (representing the colonization-phase seabed) is characterized by the lowest sedimentation rate. T1’s top position corresponds to the highest sedimentation rate. Normal stacking of deeper-tiering ichnofabrics upon shallower-tiering ones indicates enhanced bottom-current velocity, diminished sedimentation, and prolonged colonization windows. Reverse stacking suggests weakened currents, accelerated sedimentation, and truncated inhabitation. Critically, placement of the shallower-tiering ichnofabrics above, with abnormal cross-cutting relationships, reveals that intervening colonizations experienced current erosion or bypass, then following with velocity deceleration. These interpretations are corroborated by flow-velocity proxies including grain size parameters (e.g, SS mean) and elemental ratios (e.g., Zr/Rb and Ca/Fe). This finding enhances ichnofabrics-based paleoenvironmental interpretations and provides a centennial-to-millennial temporal link bridging geological (ten-thousand to million-year) and biological (daily to annual-scale) timescales.