Phosphatic Sediments and Rocks Recovered from the Peru Margin during ODP Leg 112

Abstract

Coring during Ocean Drilling Program (ODP) Leg 112 recovered phosphatic materials from six sites in forearc basins on the shelf and upper slope west of Peru. Three types of phosphates occur. F-phosphates are friable, light colored micronodules and peloids of carbonate fluorapatite (CFA); these formed by precipitation of CFA in laminated diatom muds deposited within the oxygen-minimum zone. Phosphatic sands, termed P-phosphates, consist of phosphatic peloids, coated grains, and fish debris, often having an admixture of fine siliciclastic grains. These sands occur in thin layers and burrowed beds up to 2 m thick; they may record high energy conditions and, in places, they occur above unconformities. Most abundant are dark and dense phosphates, herein called D-phosphates, that occur as nodules, gravels, and hardgrounds. These phosphates formed through complicated cycles of CFA precipitation during early diagenesis, erosion and exhumation, and reburial and rephosphatization processes associated with changing energy conditions that may reflect the effects of changes in sea level on the Peru shelf. CFA cements in P-and D-phosphates often replaced microbial structures, but our data do not reveal whether this microbial involvement was passive or active. The three phosphate types record a time and energy spectrum, with F-phosphates at the lower end, D-phosphates at the upper. Phosphates along the Peru margin occur in sediments as old as middle Miocene but are most abundant in Pliocene and especially Quaternary upwelling sediments. F-phosphates are most common in deeper water, outer-shelf/upper-slope sites, whereas D-and P-phosphates tend to predominate at shallower shelf sites more subject to episodic high energy conditions, especially during lowstands of sea level. Although lowstands of sea level probably led to concentration of phosphatic particles by winnowing, most CFA precipitation and cementation may have occurred, at least during Quaternary time, during interglacial highstands.

Phosphogenesis and active phosphorite formation in sediments from the Arabian Sea oxygen minimum zone

“…Many of these phosphorite deposits have been identified as lag deposits (Kolodny, 1981;Garrison and Kastner, 1990). Beside redeposition processes, winnowing has been suggested to play an important role in the formation of phosphorites (e.g.…”

“…The age difference between "clean" and "coated" foraminifers observed at both 2 and 24.5 cm depth implies that the downslope transport of phosphatised particles was a gradual process. The spherical and rounded pellets could be an indication for reworking (Glenn et al, 1994), but their smooth surfaces could also be explained by an external development of microbes around the grains (Garrison and Kastner, 1990).…”

Records of past mid-depth ventilation: Cretaceous ocean anoxic event 2 vs. Recent oxygen minimum zones

“…The sediments showed 0.3-0.6 m thick intervals of laminated and sub-laminated sediments with intercalated homogenous bioturbated units. They are unconformably overlain by sand-rich layers with phosphorite pebbles representing periods of erosion due to strong near-bottom currents (Reimers and Suess, 1983;Garrison and Kastner, 1990). In banded core sections, the laminae form 0.3-0.7 mm thick couplets of clay-rich and siltrich layers probably reflecting depositional variability on seasonal timescales.…”

Textures, Microstructures, and Lithologies of Japan Sea Sediments from the Lower Portion of Hole 799B, Leg 128

“…This phosphatic material is not as pure as the "F-phosphate" nodules described below, but it consists of silica or siliceous claystone enriched with phosphate to produce the typical reddish-brown color and birefringence of the "F-phosphates." The features we term "D-phosphates," following the classification of types of phosphatic material of Garrison and Kastner (1990), are dense, hard silt or sand-sized nodules (PI. 4, Fig.…”