The early late Paleocene event at Shatsky Rise can be identified by comparing the changes in faunal compositions observed in the planktonic foraminiferal assemblages between holes, the magnetic susceptibility, and the presence of phillipsite. A sequence of steps can be recognized, in stratigraphic order from older to younger (Fig. F7):
The onset of the event seems to occur at the same time in sections from all holes, although the signal is stronger in the shallower holes (1209A and 1210A), where the magnetic susceptibility values are higher (80–100 x 10–5 SI) and the clay-rich layer is thicker (Fig. F7; Table T1). In these shallower holes, the beginning of the sharp increase in magnetic susceptibility values, the deposition of phillipsite, and the change in faunal composition are almost coincident.
Conversely, in the deeper holes (1212B and 1211B) the sequence of temporal steps is less clear, as the beginning of the sharp increase in magnetic susceptibility values seems to precede the deposition of phillipsite. In Hole 1211B, the delay in the deposition of phillipsite is the result of mixing, as shown by the planktonic foraminiferal assemblages, so reworking obscures the record of the onset of the event (Fig. F7). On the contrary, in Hole 1212B, characterized by lower magnetic susceptibility values and a less dissolved planktonic foraminiferal assemblage, the onset of the event occurs gradually: the beginning of the sharp increase in magnetic susceptibility seems to precede both the deposition of phillipsite and the faunal change. Moreover, the lower abundance of phillipsite in Hole 1212B compared to the other holes might be related to a different depositional environment, as is also suggested by the lighter color of the sediment (see Fig. F7).
The end of the event in all holes occurs more gradually than the onset and coincides with a decrease in magnetic susceptibility, the disappearance of phillipsite, and a sharp increase in the abundance of I. albeari.
The dissolution event registered by the planktonic foraminiferal assemblages is supported by a decrease in carbonate content (Bralower, Premoli Silva, Malone, et al., 2002) and the presence of the clay-rich interval containing phillipsite and fish teeth. Phillipsite is interpreted as an alteration product of tephra (volcanic ash), especially basaltic glass, at the seafloor or during shallow burial; it replaces volcanic material in slow-sedimentation environments (Rothwell, 1989; Kastner, 1999). Therefore, the presence of abundant crystals of phillipsite and fish teeth suggests slow sedimentation or intervals of seafloor exposure. This is consistent with the pervasive dissolution of carbonate, which occurs when sites are located close to the lysocline depth range or close to the carbonate compensation depth (CCD).
Biostratigraphy suggests that the early late Paleocene event (~58.4 Ma) is of short duration (<1 m.y.). The thin clay-rich layer found in this interval is comparable to the Paleocene/Eocene Thermal Maximum (PETM) (~55.5 Ma); both events involved abrupt shoaling of the lysocline and CCD. PETM carbonate dissolution likely resulted from bottom water oxidation of methane released from sedimentary methane hydrate reservoirs to CO2 (Dickens et al., 1995, 1997); the cause of the early late Paleocene event is unknown and further investigation is required. At Shatsky Rise, these short-term events seem to be superimposed on a long-term record of CCD variation. In fact, the Paleogene sedimentary record from Sites 1209–1212 is strongly cyclic. The identification of low-amplitude fluctuations in the magnetic susceptibility and color reflectance records highlights the occurrence of episodic dissolution events resulting from periodic lysoclinal and CCD shoaling that perturbated Paleogene carbonate deposition. Such cyclicity suggests the possibility of orbitally controlled lysocline depth changes, probably related to variations in surface water productivity and/or deepwater mass composition (Bralower, Premoli Silva, Malone, et al., 2002).
In the studied samples the original faunal composition is overprinted by dissolution, so no clear evidence of variable surface water productivity is observed. The hypothesis of a shoaling lysocline fits well with the decrease in thickness of the clay-rich layer with depth (Table T1), which indicates more condensed sedimentation in the deeper sites that were closer to the CCD. The occurrence of an environmental perturbation is also confirmed by the composition of the planktonic foraminiferal assemblages. For instance, the absence of both surface- and thermocline-dwelling taxa in the clay-rich layer (except for the dissolution-resistant igorinids) is a reasonable consequence of significant shoaling of the lysocline, so a high percentage of taxa dissolved while they settled through the water column. The assemblage changes might also be the result of changing deepwater circulation and shoaling of the boundary between the warm surface water and the deep cold water, the latter being more corrosive.
The assemblages consist mainly of low-latitude tropical and subtropical symbiont-bearing igorinids, which are believed to have occupied a depth habitat between the shallower photosymbiotic morozovellids and the deeper asymbiotic subbotinids (Berggren and Norris, 1997). However, the abundance of igorinids, as well as the common occurrence in the shallower holes of I. pusilla "high trochospire" and I. albeari "chubby" (morphotypes of I. pusilla and I. albeari, respectively), could be interpreted as a morphologic response to a decrease in surface water productivity. Reduced nutrient supply could have caused an increase in species competition so that the igorinids modified their morphologies to the nutrient-limited mesotrophic environment. Moreover, such faunal variations might reflect the difficulty of adapting to changes in water mass owing to modification of deepwater circulation that enhanced dissolution on the seafloor. Additional data on the ecological affinities of the igorinids species, especially I. albeari, require further study.
In conclusion, high-resolution quantitative analysis of the planktonic foraminiferal assemblages in the interval surrounding the clay-rich ooze layer reveals significant changes in faunal composition, even though the original faunal composition was altered by dissolution, which affected a large section of the water column. A possible cause of the increased dissolution on the seafloor could be attributed to a change in carbonate solubility, probably related to changes in deepwater circulation over Shatsky Rise and/or variation in surface water productivity, either of which could have caused an abrupt shoaling in the depth of the lysocline and CCD.