Paleocene and Eocene sediment sections at Sites 1209 and 1211 are relatively expanded and complete (Fig. F3). As suggested by the various proxy records for dissolution (Figs. F4, F5, F6, F7), this probably reflects a history where the two sites remained above the lysocline for good portions of the early Paleogene. Given estimated paleodepths of 2.0 km for Site 1209 and 2.5 km for Site 1211, this is consistent with a Pacific CCD deeper than 3.0 km during most of the Paleocene and Eocene (van Andel, 1975; Lyle, Wilson, Janacek, et al., 2002).
However, intervals of carbonate dissolution do punctuate the Paleocene and Eocene sediment sections on Shatsky Rise. When all four dissolution indexes are examined collectively, prominent dissolution occurred at both sites during the late Paleocene (lower Zone P4, ~59–58 Ma), and during the middle to late Eocene (Zones P11–P16, ~45–33.7 Ma). We discuss these two "dissolution episodes" below but note that intervals of enhanced carbonate dissolution also occurred during the late Paleocene (upper Zone P4, ~56 Ma), and the late early Eocene (Zone P7, ~51 Ma). The first of these other dissolution intervals immediately precedes the IETM. Interestingly, however, the IETM does not stand out in our records, as discussed below.
Considering the entire time interval examined, times of dissolution generally seem longer and more pronounced in Hole 1211A compared to Hole 1209A (Figs. F4, F5, F6, F7). This is consistent with the depth positions of the two sites.
Beginning close to the boundary between the P3 and P4 foraminiferal biozones (~59.1 Ma) and continuing for 1 to 2 m.y. afterwards, BENTH and FRAG are unusually high at both Site 1209 and Site 1211 (Figs. F6, F7). There are also noticeable drops in the carbonate content and coarse fraction of bulk sediment (Figs. F4, F5). At both sites the dissolution episode closely coincides with the mid-Paleocene biotic event (Figs. F4, F5, F6, F7). Note that Petrizzo (this volume) dated the mid-Paleocene biotic event at ~58.4 Ma whereas the planktonic foraminiferal biostratigraphy used in this study (Berggren et al., 1995; M.R. Petrizzo, pers. comm., 2004) gives an age of ~59.1 Ma. The biotic event is characterized by high magnetic susceptibility, high abundances of phillipsite and manganese-coated foraminifers, and residual foraminiferal assemblages (Shipboard Scientific Party, 2002; Petrizzo, this volume). These sedimentological features further support carbonate dissolution and reduced sedimentation rates (Shipboard Scientific Party, 2002; Petrizzo, this volume). Thus, all information suggests that the lysocline shoaled significantly during this time.
Importantly, carbonate dissolution as measured in our study extends several meters beyond the biotic event as defined by obvious lithological change. For example, in Hole 1209A, the zone of high magnetic susceptibility and phillipsite spans from ~212.30 to 211.85 mbsf, whereas BENTH and FRAG generally exceed 25% from 215.80 to 210.90 mbsf. Thus, it appears that the biotic event marks a brief extreme within the context of a longer, 1- to 2-m.y. interval of unusual conditions. The relatively long time supports the view of Petrizzo (this volume) that the biotic event and its coeval carbonate dissolution were somehow linked to a change in ocean circulation, rather than a massive input of carbon such as occurred at the IETM. We note that an unusual black shale facies closely corresponds to the start of the P4 foraminiferal biozone in sections deposited off the east coast of New Zealand and that this also has been attributed to a change in Pacific circulation (Hollis et al., 2005).
All indexes for dissolution also suggest a relative shoaling of the lysocline in the late Eocene beginning at ~45 Ma and becoming particularly prominent between 37 and 33.7 Ma. Support for such dissolution on Shatsky Rise comes from sedimentation rates, which drop significantly across this horizon at Sites 1209, 1210, 1211, and 1212 (Bralower, Premoli Silva, Malone, et al., 2002).
Site 711 on Madingley Rise in the Central Indian Ocean also contains a condensed interval of low carbonate accumulation during the late Eocene (Peterson et al., 1992). This may hint at a correlative shoaling of the lysocline in this basin. Furthermore, existing CCD records for the Pacific and Indian Oceans (van Andel, 1975; Lyle, Wilson, Janacek, et al., 2002) indicate an anomalously shallow CCD of ~3.2 km for the late Eocene. Although the exact shapes of the CCD and lysocline through the early Paleogene remain poorly constrained, both probably shoaled between 45 and 33.7 Ma, perhaps to minimum depths between 37 and 33.7 Ma.
The magnitude and longevity of this middle to late Eocene dissolution interval suggests a change in the ocean carbonate budget, which may ultimately relate to alkalinity and Ca inputs. Interestingly, this time interval was characterized by significant cooling and the first ephemeral ice sheets on Antarctica (Zachos et al., 1994, 2001). One explanation is that this represents a time of relatively high sea level but reduced weathering, so that the limited inputs were predominantly stored on the shelves.
At Sites 1209 and 1211, the IETM (~55.5 Ma in this study) is marked by an ~12-cm layer of clayey nannofossil ooze with a sharp basal contact, a 1-mm clay layer, and a gradational upper contact (Shipboard Scientific Party, 2002). Additionally, nannofossil preservation decreases toward the base of this unit (Shipboard Scientific Party, 2002). The lysocline probably shoaled significantly and rapidly during the IETM on Shatsky Rise (Colosimo et al., this volume) and in all regions of the ocean in response to massive carbon addition (Dickens et al., 1997; Dickens 2000).
Our records do not express this lysocline shoaling very well, probably for two related reasons. First, the IETM is poorly represented in Holes 1209A and 1211A (e.g., core recovery and splitting appear to have disturbed the clay seam in Hole 1209A) (Shipboard Scientific Party, 2002). Second, we did not examine the base of the interval with appropriate sample resolution. Interestingly, though, BENTH and FRAG both drop significantly within 10–20 cm above the start of the IETM (Figs. F6, F7). This interval also has a corresponding influx of well-preserved planktonic foraminifers, which increase the >38-µm fraction (Fig. F5). Together, these observations may indicate a deepening of the lysocline immediately following the initial lyscoline shoaling and carbon addition. Carbon cycle models that include a weathering feedback, where increased CO2 in the atmosphere accelerates continental weathering, predict such a change in the lysocline (Dickens et al., 1997).