During deposition of the OAE1a interval, the Shatsky Rise was likely an isolated submarine high in the equatorial region of the central Pacific. Benthic foraminiferal evidence suggests that Sites 1207, 1213, and 1214 on the Shatsky Rise lay at middle to lower bathyal depths (500–2000 m) during the Aptian (Bralower, Premoli Silva, Malone, et al., 2002). Therefore, of the depositional processes associated with black shale accumulation summarized by Stow et al. (2001), pelagic settling was perhaps the most important. Pelagic settling would have likely taken the form of pellets created by zooplankton in highly productive surface waters (Stow et al., 2001). Lamination within these sediments could be attributed to bacterial mat development, bottom current activity, or perhaps planar burrowing (pseudolamination, as described in Schieber, 2003). What appear to be whole fish remains in the organic-rich intervals support the idea that bottom waters were poorly oxygenated.
Larson and Erba (1999) argue that there should be an increase in carbonate dissolution with the onset of OAE1a. Bralower et al. (1993) suggest that low carbonate contents of OAE sections as reported in Tethyan (Coccioni et al., 1992; Premoli Silva et al., 1999) and Mid-Pacific Mountain (e.g., Dean et al., 1981) sections are likely related to primary productivity, rather than a secondary byproduct of diagenetic dissolution associated with organic matter oxidation. The radiolarian sparite observed below the OAE1a interval at Site 1207 and also at Site 1214 (Table T1) suggests there may have been significant carbonate mobilization in the section. The expanded texture and beautiful preservation of delicate spiny radiolarian tests encased in spar (Fig. F7) suggest calcite precipitation must have been a very early phenomenon, prior to sediment compaction. Volume estimates of 94% calcite from XRD data for one sample (Table T2) attest to the original high intraparticle (within radiolarians) and interparticle porosity prior to cementation (Fig. F7). It is unclear if this calcite was derived from the dissolution of calcareous microfossils in the overlying organic interval or if carbonate directly precipitated near the sediment/water interface prior to the OAE1a. As stated above, no petrographic evidence for foraminifers was observed within the OAE1a organic-rich intervals, not even ghosts.
Authigenic carbonate is also present at Site 463. Thin sections prepared from the Site 463 cores for this study show significant amounts of authigenic Fe stained carbonate (calcite according to Dean et al., 1984), but as matrix replacement rather than sparry cements. Differential diagenesis across the OAE1a intervals at both Site 463 and the Shatsky sites may have contributed to the higher recovery rates compared to units above and below.
Other evidence for the depositional environment of the OAE1a interval comes from the nature of the organic-rich intervals. Most of the organic matter in organic-rich intervals in the Pacific is thought to be algal or bacterial with little input from terrestrial sources (Dean et al., 1984). Shipboard analyses of the OAE1a intervals showed them to be composed of Type I organic matter of algal and bacterial origins, including evidence for some produced by cyanobacteria and haptophytes (e.g., coccoliths) (Bralower, Premoli Silva, Malone, et al., 2002). The latter indicate production of at least some of the organic matter in the photic zone. Shipboard scientists (Bralower, Premoli Silva, Malone, et al., 2002) argued that deposition as microbial mats in highly disoxic or anoxic conditions were needed to explain the nature of the bacterial organic matter, the anomalously high percentage of organic matter, and the overall excellent preservation of the organic compounds.
Shipboard scientists noted in their core descriptions that the organic-rich shales at Site 1213 differed from those at Site 1207 in that the Site 1213 samples showed no evidence for lamination and they were moderately to intensely bioturbated (Fig. F7). This could be a function of preferential recovery given the moderate recovery rates (50% and 30%, as mentioned earlier). Where intact, the organic-rich shales have a gradational base (over several centimeters) but an abrupt top. The presence of bioturbation suggests that oxygen levels were not sufficiently low to completely inhibit benthic activity. Even so, total organic carbon (TOC) values ranged up to ~25 wt% in the Site 1213 core fragments.
The petrographic observations made across the well-developed OAE1a intervals in Sections 198-1207B-44R-1 and 198-1213B-8R-1 indicate that often the organic matter has a more globular texture (sapropellic algal matter) that progressively becomes more laminated upsection. This likely represents the onset of organic matter accumulation and the resultant development of bacteria colonies on the sediment surface. These laminae are first irregular and discontinuous (wispy) (e.g., Fig. F7), and then become more continuous and dense (e.g., Fig. F6). No evidence for current reworking of cohesive mats (e.g., roll-up structures of Simonson and Carney, 1999) was observed in the cores, but given the bathyal depth at which these sediments accumulated, this is not surprising. Further examination of OAE1a samples (pristine, not impregnated with blue epoxy as in this study) using a scanning electron microscope would be beneficial to understanding the origin of these laminae, perhaps providing conclusive evidence for mat development (i.e., preserved remains of mat-building organisms in life position) (Schieber, 1999).
Surface-grazing organisms or bioturbative mixing could have produced discontinuities in the mat. Observations of modern samples of submarine mats show them to contain a limited indigenous meiofauna of foraminifers and nematodes (Grant, 1991). Leckie et al. (2002) suspect that the smaller planktonic foraminifers associated with OAEs might have been bacterial grazers. But as discussed above, there was no direct evidence of foraminifers within the OAE1a interval on the Shatsky Rise. Discontinuities could also be a product of the geometry and size of the bacterial mats. Direct submarine observations of seasonally developed bacterial mats from the floor of the Santa Barbara Basin show them to occur as patches a few centimeters to meters in scale (Grant, 1991). Photographs indicate that they grade laterally into burrowed dysaerobic sediment (Grant, 1991). Thus, the net product of these variabilities acting through time, especially if mat distributions fluctuate and are overprinted by bioturbation, could be mostly wispy textures. On the Shatsky Rise, the flattened and stringy clay-rich patches in the OAE1a wispy facies (Fig. F7) could be fecal in origin. In modern deepwater mats the filamentous bacterial sheaths can bind sediment, including fecal pellets and microfossils, to form a spongy texture (Williams and Reimers, 1983). If originally calcareous (nannofossiliferous), such pellets may have been diagenetically diminished during burial and compaction, with only dissolution residues remaining like deflated balloons. "Interrupted" microlaminations were interpreted by Timofeev et al. (1981) as the products of precipitation from an organic mineral hydrogel, producing montmorillonitic clay minerals and organic seams, strands, or lenses that mimic lamination. Finally, the wispy textures may be fundamentally inherent to mat environments rather than a grazing or pelletal phenomenon, in that such structures are even reported in Archaean carbonaceous cherts where bioturbation and pelletization are not options for their formation (Walsh and Lowe, 1999). Thus, these wispy and discontinuous textures may be of multiple origins. There is evidence for bioturbation in all but the most densely laminated sections, suggesting that anoxia was periodic. Laminated sediments would only survive in the most severe periods of anoxia. Higher in the Site 1207 OAE1a section, where the laminations become more continuous, the presence of whole fish fossils suggests more pronounced anoxia. The structure of these well-defined laminations in the upper OAE1a at Site 1207 is a function of variations in the density of organic matter, which could represent seasonal fluctuations in organic matter accumulation.
The OAE1a intervals at Sites 1207 and 1213 are ~600 km apart, suggesting that some of the differences between the sections may be attributed to latitudinal variability in oceanic productivity. Furthermore, Site 1207 on the Northern High is estimated to have been at a shallower depth (1.3 km) than sites on the Southern High (e.g., 2.8 km at Site 1213) during deposition of the OAE1a interval (Bralower, Premoli Silva, Malone, et al., 2002). This depth difference could be significant in terms of their relative positions with respect to the carbonate compensation depth (CCD) (Bralower, Premoli Silva, Malone, et al., 2002). Because of the less than perfect recovery of the OAE1a interval (30%–50%), it is difficult to determine whether observed differences between the sections at Sites 1207 and 1213 have been made on truly representative or biased sample sets.
There are three organic-rich intervals at Site 463, described as having abrupt bases and gradational tops (Dean et al., 1981). The nature of the organic material recovered at Site 463 is similar to that from the Shatsky Rise (Bralower, Premoli Silva, Malone, et al., 2002). There are several detailed studies of the organic material at Site 463. Timofeev and Bogolyubova (1981) describe granular sapropelic (algal) material similar to the globular (round, <30 µm) material observed at Shatsky, as well as structureless organic–sediment mixtures that they call sapro-collinite. They also term "peculiar" the strandlike to bandlike texture that imparts the horizontal texture (not lamination) to the sediment. The few organic-rich sedimentary rocks from Site 463 examined here (Table T1) are radiolarian porcellanites to claystones with globular (round, <30 µm) to stringy discontinuous organic matter similar in texture to that observed on Shatsky Rise. Petrographic differences between the Shatsky and Site 463 OAE1a intervals found during this study (Table T1) include locally significant replacement of radiolarians by pyrite (Fig. F8) and late-stage replacement by Fe-stained calcite.
OAEs have been variously linked to volcanism. Vogt (1989) suggested that enhanced spreading rates and higher heat flow at the mid-ocean ridges would have warmed ocean bottom waters, eventually resulting in higher productivity. Alternatively, warmer bottom waters may have originated in marginal seas flooded during sea level rise bought about by increased ridge volumes associated with rapid spreading (Arthur et al., 1987). In the Pacific realm, the linkage is also manifested in the close association of the OAE1a with volcanic ash beds (Dean et al., 1984; Sliter 1989, 1999) and the close temporal linkage of the OAE1a to the large pulse (superplume?) of off-ridge mafic volcanic activity that created the Ontong Java and Manihiki Plateaus (Fig. F2) (Larson, 1991b; Larson and Erba, 1999; Tarduno et al., 1991).
The inferences are that the volcaniclastic intervals would have been produced by eruption of these large igneous provinces. However, the OAEs are present on what were then submerged topographic highs (Sliter, 1989) that would have been isolated from direct submarine ash input. This means that the ash must have either been supplied through the water column by air fall or that the ash would have been produced by volcanic centers that were active on each of the highs.
Several questions arise as to the origin of the volcaniclastic intervals associated with the OAE1a on the Shatsky Rise and elsewhere in the Pacific:
There are three possible sources: (1) a magmatic arc to the east; (2) OJP volcanism; and/or (3) volcanism on the Shatsky Rise. Each of these sources might have supplied texturally and compositionally distinct volcanic debris. Deciphering the origin of these ash beds is made more complex because of their intense alteration, including dissolution of glass and mineral grains and precipitation of authigenic phases. However, blue-dyed epoxy impregnation of primary and secondary pores allows for the recognition of textural attributes of the debris (e.g., Fig. F5) that shed some light on their composition (mafic to felsic) and likely origin (epiclastic, hydroclastic, or pyroclastic). The feasibility of each source is discussed below.
The Aptian is associated with higher rates of seafloor spreading (e.g., Larson, 1991a, 1991b), which implies that rates of subduction and subduction-related magmatism should have also increased around the paleo-Pacific during this time period. Paleogeographic reconstructions place the Shatsky Rise in the equatorial central Pacific, which limits possible source volcanoes to the Central American or northern South American magmatic arcs, which today are the main source of ash in the equatorial central Pacific (Fisher and Schminke, 1984). Far-traveled ash from arc eruptions would have likely consisted of silicic bubble-wall shards and highly to moderately vesiculated glass, similar to that which accumulated in the Cenozoic section as the Shatsky Rise traveled closer to Asian arc sources (Gadley and Marsaglia, this volume). These textures were not dominant in the OAE1a-related ash intervals, so an arc-related source seems improbable.
Early drilling on the OJP suggested that it formed rapidly during the early Aptian (Tarduno et al., 1991). Thanks to ODP Leg 192 (Mahoney, Fitton, Wallace, et al., 2001; Mahoney et al., 2001), more is now known about the basement and cover rocks of the OJP. Biostratigraphic and geochronologic data from this leg and other studies of outcropping equivalents indicate the plateau was produced in a brief magmatic pulse at ~120 Ma (Mahoney et al., 2001). Shipboard scientists concluded that the plateau was shallow in the Aptian, but other data suggest that it was produced by submarine eruptions (Roberge et al., 2003). At Site 1183 on the OJP (Mahoney, Fitton, Wallace, et al., 2001), 2 m of volcaniclastic turbidite sands consisting of partly glassy basaltic rock fragments and altered brown blocky to moderately vesicular glass shards were recovered above pillowed basalt flows.
Mafic volcanic centers are not known to produce much air fall ash, but Tarduno et al. (1991) suggest that formation of the OJP would have produced "exceptional eruption of ash." They believe this ash is present at Site 167 on a topographic high northeast of the OJP. More recently, Ingle and Coffin hypothesized that submarine basaltic magmatism on the OJP was the product of an extraterrestrial bolide impact that also contributed to the formation of the OAE1a (Coffin and Ingle, 2003; Ingle and Coffin, 2003, 2004). Such an event would also likely first produce pyroclastic ejecta, possibly glassy spherules, then induce voluminous eruptions and production of more pyroclastics.
As shown by biostratigraphic studies (e.g., Erba, 1994), the OAE1a interval slightly postdates the OJP-building event. Thus an OJP source can only be called upon for the ash beds that directly underlie the OAE1a intervals. These ash beds are very altered and mainly consist of birefringent clay (smectite) with traces of plagioclase feldspar. The alteration has obscured any textural information that might have linked them to an OJP. A more parsimonious interpretation is that they were produced by proximal eruptions on the Shatsky Rise.
Elsewhere in the western Pacific Ocean, including the Mid-Pacific Mountains, an extensive mid-Cretaceous magmatic event is associated with widespread seamount production and intrusive complexes (Schlanger et al., 1981). This magmatism is thought to have been induced by the Ontong Java event (Tarduno et al., 1991). Previous workers have interpreted the ash beds at Site 463 to be a mixture of epiclastic and pyroclastic debris (Dean et al., 1984); admixed shallow-water fauna led Vallier and Jefferson (1981) and Timofeev et al. (1981) to propose that the source of the volcaniclastic material was a volcanic island. The composition of the alteration products led most to believe that the ash was mafic; however, some proposed a felsic to alkalic component (Hein and Vanek, 1981; Mélières et al., 1981). My observations of thin sections from several tuffaceous intervals at Site 463 (Fig. F8) support a mafic source of volcaniclastic debris above and below the OAE1a horizon. Such a local origin is also plausible for the tuffaceous beds on the Shatsky Rise.
Shatsky Rise is a large (1200 km x 400 km) volcanic plateau that rapidly formed in association with a complex oceanic ridge-ridge-ridge triple junction during the Jurassic–Early Cretaceous (Sager et al., 1988; Nakanishi et al., 1999). Basement drilling has been unsuccessful and there are many unresolved questions as to the origin of the rise (Bralower, Premoli Silva, Malone, et al., 2002; Sager et al., 1988; Nakanishi et al., 1999). Previous workers have seen no evidence for Cretaceous volcanism on the Shatsky Rise associated with the Ontong Java event at 120 Ma (Nakanishi et al., 1999). However, diabase sills intruded into Berriasian sedimentary rocks at Site 1213 (Bralower, Premoli Silva, Malone, et al., 2002) indicate post-Berriasian magmatism on the Southern High dated by Mahoney et al. (2005) at ~144 Ma.
The distribution of ash on the Shatsky Rise, above and below the OAE1a interval, is similar to that observed at Site 463. The ash intervals that occur below the OAE1a interval on the Shatsky Rise are texturally indeterminate (discussed above) but contain no relict microlitic or tachylitic debris, just some vague vesicular textures. In contrast, the ash beds above the OAE1a interval at Site 1207 consist of a mixture of altered blocky to slightly vesicular vitric components of likely hydroclastic origin and epiclastic debris. The epiclastic volcanic grains are subangular to well rounded, include microlitic and tachylitic fragments, and are associated with glauconite (Figs. F5, F6). The microlitic textures and presence of tachylite suggest that these fragments were derived from more slowly cooled (subaerial) basalt flows (see discussion of mafic fragment types in Marsaglia, 1992, 1993). These results suggest that there was a subaerially exposed volcanic edifice on the Northern High soon after OAE1a deposition. Undated rudist- and coral-bearing shallow-marine limestones were recovered from the Southern High, but these could be as old as Jurassic (Sager et al., 1999).
Glauconite is significant as it is generally thought to be produced in modern middle-shelf (50 m) to upper-slope (450 m) environments where sediment accumulation is slow but in ancient systems may have formed in much shallower water (Chafetz and Reid, 2000). At Site 1207, glauconite appears to have been transported along with the epiclastic volcanic debris. Elsewhere in the Pacific, glauconite has been described in the shallow cover sequences of seamounts (e.g., Paleocene at Site 432 on Nintoku Seamount by Marsaglia et al., 1999). Jeans et al. (2000) describe Aptian–Albian glauconitic deposits that formed by the replacement of mafic volcaniclastic sand/silt in marine settings near the island and massif volcanic centers that formed in response to rifting and extension in the North Sea.
As mentioned previously, the OAE1a interval occurs during a period of eustatic sea level rise (Bralower et al., 1993). Thus, the sandy glauconitic and epiclastic beds cannot be attributed to eustatically driven shelf exposure whereby sand was transported into deeper water. Shelf development must instead be linked to volcanic topography. The limitation of the glauconite to directly above the OAE1a interval indicates that, once formed, the topographic highs responsible for these sediments must have then rapidly deepened, perhaps as a response to thermal subsidence compounded by the effects of rising sea level.