PALEOENVIRONMENTAL INTERPRETATIONS

Cores 210-1276A-95R and 94R contain very common calcareous nannofossil fertility index species such as D. rotatorius, Biscutum spp., and Zeugrhabdotus moulladei, suggesting that the enriched organic matter in the laminated black shales is at least partially the consequence of elevated productivity. Cores 210-1276A-95R and 94R are also characterized by high abundances of B. constans, D. rotatorius, and small Zeugrhabdotus spp. These taxa are well known from the literature to be indicators of high fertility of surface waters (Roth and Bowdler, 1981; Premoli Silva et al., 1989; Coccioni et al., 1992; Erba et al., 1992).

This laminated black shale interval likely correlates with the "Niveau Paquier" black shale of the Vocontian Basin in southeast France (Bréhéret, 1994). Elsewhere it is known as OAE1b (e.g., Erbacher et al., 1996; Leckie et al., 2002). If so, this is one of the deepest-water examples of OAE1b known, and it is also one of the thickest (e.g., Erbacher et al., 1998, 1999, 2001). Nannofossil evidence shows that high-fertility conditions characterize the black shale interval.

No benthic foraminifers or other evidence of benthic organisms, including burrows, were found in the laminated black shale of OAE1b at Site 1276, suggesting that the deepest parts of the central North Atlantic were anoxic at this time. This finding complements data based on membrane lipids (biomarkers) extracted from organic matter preserved in an OAE1b black shale from the western North Atlantic suggesting that much of water column may have been anoxic during this event because of a massive expansion of chemoautotrophic Archaea (Kuypers et al., 2001).

The paucity of calcareous planktonic and benthic foraminifers recovered from sediments in Cores 210-1276A-97R through 75R suggests that the depositional environments were below the CCD and probably at abyssal depths and that the rare specimens recovered are actually a product of debris flows and turbidity currents. Relatively abundant and diverse assemblages of agglutinated foraminifers indicate that bottom conditions were possibly conducive to these assemblages, and, although generally small, the characteristic genera recovered are representative of deeper-water dwellers such as Ammodiscus spp., Rhizammina spp., and Glomospira spp. (Kuhnt and Urquhart, 2001). The distinct paucity of burrows indicates that conditions were not hospitable for all bottom-dwelling organisms.

The presence of abundant radiolarians throughout lithologic Subunit 5C between Samples 210-1276A-97R-4, 66–70 cm, and 76R-6, 118–122 cm, the rare but consistent occurrence of fish teeth, and the fact that calcareous nannofossils were present in surface waters suggest that surface waters and probably the upper part of the water column were subject to normal open-marine conditions during this interval.

Common wood and plant debris recovered consistently through this sedimentary subunit (1706.57–1502.12 mbsf) and throughout the Albian sediments until upper Albian Sample 210-1276A-51R-4, 66–70 cm, in Subunit 5B above, suggest that although the seafloor was at abyssal depths the site was exposed to strong influence from the adjacent land mass. It also suggests that the prevailing climate supported abundant vegetation and that there was sufficient run-off to transport large quantities of this material into the marine realm.

The upper Albian at Site 1276 (Cores 210-1276A-48R through 41R) is characterized by moderately preserved and rare to moderately abundant assemblages of planktonic foraminifers. The greater abundance of planktonic foraminifers parallels generally higher concentrations of carbonate along continental margins during the late Albian, as well as larger sizes and more calcified species of planktonic foraminifers at this time (Leckie et al., 2002). A greater flux of carbonate associated with increased carbonate productivity in the near-surface ocean can buffer the carbonate solubility of the deep ocean, thereby enhancing carbonate preservation in an otherwise carbonate-poor environment (e.g., McCarthy et al., 2004). In addition to greater abundance and improved preservation, not all uppermost Albian assemblages of Site 1276 show the pronounced size sorting observed in the upper lower to lower upper Albian interval, therefore suggesting that the pelagic rain of carbonate reached the seafloor during latest Albian time.

Organic carbon–rich black shales in the upper Albian of Hole 1276A (Tucholke, Sibuet, Klaus, et al., 2004) may be correlative to other suspected OAEs (OAE1c in the upper Albian and OAE1d in the uppermost Albian) (Leckie et al., 2002; Arnaboldi and Meyers, this volume). These black shales may reflect times of enhanced productivity in surface waters and deposition under anoxic seafloor conditions. The abundance and small size of the radiolarian assemblages recovered could be indicative of enhanced productivity at the surface and upper reaches of the water column. Conversely, the limited size range of the specimens may be due to size sorting and redeposition by bottom currents. The latter hypothesis is somewhat supported by the consistent and relatively abundant occurrences of calcareous nannofossils, at least up to Sample 210-1276A-42R-CC. Fish teeth and debris also become very rare above Sample 210-1276A-41R-CC. Extensive burrowing of the sediments suggests the existence of a thriving benthic community not consistent with prolonged periods of stagnant anoxic bottom conditions.

The fine-grained autochthonous sediments typically contain rare agglutinated benthic foraminifers, black carbonized plant debris, pyrite, and occasionally rare calcareous benthic foraminifers. Rare, small specimens of calcareous benthics, including Gyroidinoides cf. G. nitidus, Gavelinella spp., Praebulimina spp., and Bolivina spp. occur sporadically with the agglutinated foraminifers in the mudrocks. It is likely that these bathyal and upper abyssal calcareous taxa were transported downslope turbidity currents or other gravity flows.

The Cenomanian/Turonian boundary is associated with a laminated black shale interval in Sections 210-1276A-31R-2 and 31R-3, which likely correlates with OAE2. Radiolarians are generally not preserved in the sediments of lithologic Subunit 5A although they do occur commonly in two samples, once just below the OAE2 interval in Sample 210-1276A-33R-CC and once just above the event in latest Cenomanian–earliest Turonian Sample 210-1276A-31R-4, 8–12 cm. The presence of these radiolarian assemblages indicates that at the time of deposition the ocean surface waters must have oxygenated and conducive to planktonic organisms. The paucity of radiolarians recovered during the OAE2 interval corresponds to the findings of Musavu-Moussavou and Danelian (2006) in their studies of ODP Leg 207 Sites 1258 and 1261 at the Demerara Rise. These authors suggest "an intensification of euxinic conditions must have favored the proliferation of sulfate reducing bacteria, but was at the same time toxic to heterotrophic protozoa such as Radiolaria."

Noncalcareous mudstones interpreted as in situ hemipelagic sediments in this subunit are moderately bioturbated.

The late phase of deposition of the Bonarelli level (OAE2) is likely accompanied by surface water cooling, as expressed by the calcareous nannofossil assemblage through two distinct peaks in abundance of Eprolithus spp. (Cores 210-1276A-31R and 29R) (Fig. F4). Eprolithus spp. is quite common at high latitudes and rare in the Tethyan realm. Its preference for cooler surface water might indicate cooler surface waters arriving in the Atlantic domain. This cooling event may be widespread if not a global phenomenon because Eprolithus peaks have been reported by several authors (Bralower, 1988; Paul et al., 1999; Nederbragt and Fiorentino, 1999; Erba, 2004) at different localities, in the Tethyan and Atlantic domain, and also in the Western Interior Basin during this time interval. Also, Sample 210-1276A-30R-3, 54–55 cm, contains the FO of the calcareous nannofossil A. octoradiata. This taxon seems to be highly controlled by temperature changes (Wind, 1979; Lees, 2002), being more abundant at higher latitudes, and thus, it is also a good indicator of surface water cooling. Increasing abundances of A. octoradiata together with Gartnerago spp., another taxon preferring cooler temperatures, occur in Samples 210-1276A-30R-2, 53–54 cm, and 30R-1, 49–50 cm, thus supporting an early Turonian cooling event. Global cooling is a predicted consequence of OAE2 due to the excess burial of organic matter and lowering of atmospheric pCO₂ (e.g., Arthur et al., 1988).

The mineral content in the washed residues includes abundant fine-grained angular quartz, abundant mica, glauconite, pyrite, and phosphate nodules indicating dominance of terrestrial input and reworking of the sediments. The paucity of pelagic microfossils, including calcareous nannofossils, and very low sedimentation rates suggest sediment starvation and shoaled CCD due to high global sea level (Thierstein, 1979; Hardenbol et al., 1998). The red oxygenated sediments indicate that the deep North Atlantic was well ventilated following a possible breach of the deepwater sill separating the North and South Atlantic ocean basins (Tucholke and Vogt, 1979).

Site 1276 may have been influenced by transitional or Boreal water masses at times during the Late Cretaceous based on the presence of cool-water calcareous nannofossil taxa and the general paucity of large biserial (e.g., species of Pseudotextularia) and multiserial (e.g., Planoglobulina) planktonic foraminifers typical of Maastrichtian-age low-latitude (Tethyan) assemblages. Alternatively, dissolution during settling may have selectively removed some of these larger taxa in all but the turbiditic sandstones. A short-lived depression of the CCD (Tucholke and Vogt, 1979) or higher carbonate surface water productivity (e.g., McCarthy et al., 2004) may have provided a more favorable environment for the accumulation and preservation of planktonic foraminifers in the deep Newfoundland Basin during the latest Maastrichtian, but the richest assemblages occur in coarser-grained mudstones and very fine to fine sandstones that reflect downslope transport and size sorting.

The condensed Maastrichtian section at Site 1276 prevented the recognition of the distinct cooling/warming phases, which characterize the climatic evolution of the Maastrichtian stage (Thibault and Gardin, 2006). In particular, the M. murus acme, which lies within the M. prinsii Zone in the upper Maastrichtian, was not observed at Site 1276. This event is characterized by a strong increase of the tropical species M. murus coincident with a dramatic drop in abundance of "cool-water taxa," likely indicating an extreme warming event before the end of the Mesozoic Era, (Li and Keller, 1998; Thiabault and Gardin, in press).

Regarding the K/P boundary, if only the calcareous nannofossil assemblages are considered, the boundary may be placed in Section 210-1276A-21R-4 between 41 and 49 cm (e.g., between the last Maastrichtian assemblages and the first Danian ones). However, a multiproxy analysis based on micropaleontology, magnetic susceptibility, natural gamma radiation, and ¹³C_org stable isotope analysis indicates that the boundary is more probably located at Section 210-1276A-21R-4, 55–57 cm, coincident with the first barren interval (Gardin et al., 2005, unpubl. data). If so, Sample 210-1276A-21R-4, 56–57 cm, just above the barren interval, is a witness of lowest Danian sediments (which are usually characterized by only few Cretaceous species and high thoracospherid fragments), and the overlying samples from the carbonate turbidite contain reworked Maastrichtian assemblages (Fig. F5). This succession of assemblages is comparable to that observed elsewhere at the Cretaceous–Paleogene transition, even though the abundance of Cretaceous species above the transition at Site 1276 is remarkably high, likely due to pervasive redeposition at this site. The K–P transition at Site 1276 is very peculiar because it is one of the few relatively continuous abyssal depth sections. Although there is apparent biostratigraphic continuity, this interval is probably very condensed (reduced biozones). A small hiatus may exist across the boundary itself.

Relatively abundant and poorly to moderately well preserved large-size benthic foraminifers typical of warm, shallow-water carbonate platforms occur in the turbidite sandstones, together with fragments of other organisms of shallow-water origin, in the uppermost Paleocene–lower Eocene of Site 1276 (Cores 210-1276A-15R through 7R) (Georgescu et al., submitted [N2]). The large-size benthics have Caribbean affinities and are of two distinct ages. An elevated bank or banks adjacent to the Newfoundland Basin (e.g., southeast Grand Banks, southern Flemish Cap, and/or Newfoundland Seamounts) likely had tropical-subtropical communities of organisms during the Campanian–Maastrichtian and again during latest Paleocene–early Eocene time. What is especially interesting about these two ages of large-size benthics is (1) they occur in sediments corresponding with the PETM and the Early Eocene Climatic Optimum (EECO) and (2) both ages (Campanian–Maastrichtian and latest Paleocene–early Eocene) correspond with times of high sea level and global warmth (e.g., Hardenbol et al., 1998; Barerra and Savin, 1999; Zachos et al., 1993, 1994, 2001; Huber et al., 2002; Moran et al., 2006; Sluijs et al., 2006). It seems that paleobiogeographic provinces shifted farther northward during these two intervals, in part related to the existence of a proto-Gulf Stream that crossed the region of the Grand Banks (Georgescu et al., submitted [N2]).

The in situ hemipelagic deposits of lithologic Unit 2 are considered to have been deposited below the CCD at abyssal depths based on the paucity of planktonic and calcareous benthic foraminifers and the dominance of agglutinated benthic foraminifers.

Nannofossil assemblages throughout this section are characterized by abundant placoliths, which are poorly preserved. This is potentially indicative of a changing hydrologic regime throughout the later Eocene–earliest Oligocene that reflects colder, more corrosive bottom currents (Wood et al., submitted [N1]). Increased bottom water circulation may be related to the onset of long-term global cooling during the middle and late Eocene (e.g., Zachos et al., 2001) or perhaps new bottom or deep water sources following the connection with the Arctic Basin (Brinkhuis et al., 2006). Furthermore, the condensed interval/potential hiatus which spans ~4.8 m.y. in the lower middle Eocene is likely the consequence of a global sea level transgression coupled with the change in hydrologic regime; this is supported by a similar condensed interval at Site 398 (Blechschmidt, 1979; Wood, et al., submitted [N1]).

The depositional environment of this unit is similar to lithologic Unit 2; evidence from the agglutinated foraminifer assemblages presented by Takata (2007) as well as the near-absence of planktonic foraminifers (except in redeposited sandstones) indicates deposition at abyssal depths below the CCD.