Over the past 1.9 Ma, for both drill cores, we constrain the geochronology of the investigated sediment by tuning previously reported benthic δ18O measurements to the Lisiecki and Raymo (34) δ18O stack (fig. S1). These age models are, for the most part, similar to previously constructed age models at the shelf location (31, 32) and at the offshore location (33). For older sediment, where no foraminifera δ18O measurements were available, we used biostratigraphic and/or magnetostratigraphic information reported by the IODP/ODP sampling parties [fig. S1; (35, 59)]. Note that in the deep location, we removed the debris flow interval between ~543 and 550 mbsf to obtain an undisturbed record. For each sedimentary record, sedimentation rates were inferred at the depth of each δ34Spyr measurement by calculating the linear sedimentation rates implied by the age models.

The IODP borehole 1352 can be divided into three distinct sedimentological units. Unit I spans the Pleistocene and consists of a 680-m-thick mud-rich sediment. Detailed inspection of sedimentary facies and foraminiferal stratigraphy reveals that dominant clay sedimentation was interrupted by the deposition of several sandy mud layers, rich in planktic foraminifera and other biogenic components, during each of the major interglacial isotope stages. These coarser-grained, more biogenic sediments reflect highstand sedimentation, and contrast with the fine-grained, terrigenous muds deposited during glacial periods. The basal contacts of the coarser-grained units with the fine-grained muds are sharp, and their contacts with overlying muds are characterized by sandy mud with abundant shells and shell fragments, interpreted as a firm ground that marks the initiation of transgressive facies. Evidence for bioturbation and bioirrigation is mostly described in the muddy units, corresponding to periods of low sea level, and often intensified below the reworked or erosional uncemented firm grounds (37).

Although the transition between units I and II is located around 680 mbsf, the transition is gradual, reflecting a gradual deepening to a slope setting. Unit II contains a gradual change from uncemented calcareous sandy mud to lithified marlstone with less frequently intercalated sandstone, except at the base of the unit (Miocene) where glauconitic sandstone becomes more common. An erosional surface, known as the Marshall Paraconformity, occurs at the base of unit II (1750 mbsf), where there is an abrupt change to lithologic unit III, which is composed of nannofossil micritic limestone (hemipelagic to pelagic foraminifera) of Eocene age.

The offshore ODP borehole 1123 penetrated 633 mbsf, recovering a succession of clay-rich nannofossil ooze, chalk, and limestone. At the top, unit I corresponds to a Pleistocene drift sediment sequence, showing glacial-interglacial couplets, composed of 188 m of clayey nannofossil oozes with numerous tephra layers.

Unit II displays lithological features that are characteristic of sediment drift deposits, i.e., composite sequences of a few centimeters to decimeters in thickness showing overall negative grading from muddy through silty-sandy lenticular beds and then positive grading back to muddy drift facies. The thickness of the silty-sandy unit generally increases with depth. Silt mottles, lenses, irregular to subhorizontal layers, and cross-laminated alignments are also preserved. Unit II is sufficiently indurated to be classified as chalk, and it contains fewer tephra beds.

Unit III consists mainly of similar sediments as the above units, except that terrigenous clay is sufficiently abundant to classify the unit as a nannofossil mudstone. The identification of sedimentary features in those units is made difficult by abundant fractures, often along bedding planes or biscuiting in the drilled cores. Nevertheless, we do observe abundant sandy lenticular beds alternating with mud-rich layers, silt mottles, lenses, irregular to subhorizontal layers, and cross-laminated alignments in Miocene sediment, supporting the inference of drift sedimentation (35). The identification of sediment drifts typically relies on the interpretation of reflection seismic datasets. Spectacular, regional-scale, dune crossbedding, with sets up to 5 m thick and tens of meters long, has been identified in the seismic profile through the North Chatham Drift. These features have been interpreted to result from a semipermanent flowing current (27, 60, 61). In addition to the presence of these thick drift deposits, which commonly contain mud waves, furrows, and other active bedforms, there is abundant evidence in the seismic profiles of erosion or nondeposition in the form of scoured bedrock and marginal mots.

A singularity of unit III is the presence of a 7-m-thick chaotic assemblage of plastically deformed clasts of nannofossil chalk at ~543 mbsf, resulting from a debris flow. Below the Marshall Paraconformity, the remainder of the core is composed of unit IV, an Eocene micritic limestone that seems to be correlative with the bottom of the shelf IODP 1352.

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