An early Maastrichtian organic-walled phytoplankton

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Marine Micropaleontology 34 (1998) 1–27

An early Maastrichtian organic-walled phytoplankton cyst assemblage from an organic-rich black mud in Core Fl-533, Alpha Ridge: evidence for upwelling conditions in the Cretaceous Arctic Ocean John V. Firth a,Ł , David L. Clark b a

Ocean Drilling Program, 1000 Discovery Drive, Texas A&M University Research Park, College Station, TX 77845, USA b Department of Geology and Geophysics, University of Wisconsin, Madison, WS 53706, USA Received 12 September 1996; revised version received 28 October 1997; accepted 26 November 1997

Abstract A diverse assemblage of acritarchs, prasinophytes and dinoflagellate cysts occurs in an organic rich black mud in Core Fl-533, taken from the Alpha Ridge, Arctic Ocean. Similarities between this assemblage and others described from the Canadian Arctic allow this black mud to be dated as early Maastrichtian, based primarily on the presence of Cerodinium leptodermum, and the absence of exclusively Campanian and upper Maastrichtian taxa. The high abundance of Comasphaeridium and marine derived amorphous organic matter may indicate a shelf to upper slope environment, possibly deposited during a marine transgression. An abundance of prasinophyte algal cysts, a moderately high peridinioid=gonyaulacoid dinoflagellate cyst ratio, and high species diversity indicate high paleoproductivity, most likely associated with upwelling conditions. When interpreted within the context of two nearby biosiliceous-rich Maastrichtian cores also taken from Alpha Ridge, and from paleogeographic reconstructions, these results indicate that an upwelling region probably existed along the eastern Arctic Ocean during at least part of the Maastrichtian, and formed different sedimentary facies in shelf to slope environments, similar to facies patterns recognized in lower latitude paleo-upwelling regions.  1998 Elsevier Science B.V. All rights reserved.

1. Introduction The geologic history of the Mesozoic and Paleogene Arctic Ocean remains one of the primary gaps in our knowledge of the ocean basins. This gap is primarily because the Arctic Ocean is largely inaccessible under a permanent ice-cap. Numerous shallow piston cores have been taken from the Arctic Ocean, yet only a few of these cores have penetrated below a cover of Quaternary sediments. Mudie et al. (1986) and Clark (1988) summarized the extent Ł Corresponding

author. E-mail: [email protected]

of the data available for older Paleogene and Cretaceous sediments of the Arctic basin, and Clark et al. (1986) gave a detailed analysis of the black organic-rich muds recovered in Core Fl-533 from the Alpha Ridge at 85º05.90 N, 98º17.80 W (Fig. 1). These sediments, previously dated as late Campanian to Maastrichtian in age (Mudie et al., 1986; Clark et al., 1986 and Clark, 1988), are the subject of this study, in which we describe in detail the organic-walled phytoplankton assemblages, reassess the age determination, and give comments on the possible paleoenvironment of this locality in the Arctic Ocean.

0377-8398/98/$19.00  1998 Elsevier Science B.V. All rights reserved. PII S 0 3 7 7 - 8 3 9 8 ( 9 7 ) 0 0 0 4 6 - 7

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J.V. Firth, D.L. Clark / Marine Micropaleontology 34 (1998) 1–27

Fig. 1. Bathymetric map of the Arctic Ocean showing the Alpha Ridge and Cores Fl-533, CESAR-6, and Fl-437. 1000 m contour interval.

Palynological studies of Upper Cretaceous sediments in the circum-Arctic regions have been based primarily on land sections. Manum (1963); Manum and Cookson (1964); Felix and Burbridge (1973); McIntyre (1974, 1975); Doerenkamp et al. (1976); Lentin and Williams (1980); Ioannides (1986) and Nu´n˜ez-Betelu and Hills (1992) all reported on organic-walled phytoplankton cyst assemblages from the Canadian Arctic Archipelago and onshore parts of the Northwest Territories. Soper et al. (1976) and Nøhr-Hansen (1996) documented organic-walled phytoplankton cyst assemblages from the Upper Cretaceous and Tertiary of eastern and western Greenland, respectively. Vozzhennikova (1960, 1967) described Mesozoic and Tertiary dinoflagellates from the Siberian Arctic and other localities in the former U.S.S.R. Lentin and Vozzhennikova (1990) re-described and re-illustrated many of these taxa. Mudie (1985) and Mudie et al. (1986) have published the only palynological reports on Cretaceous sediments from the central Arctic

Ocean —the biosiliceous mud recovered from Core CESAR-6, also located on the Alpha Ridge, and brief notes on the Cretaceous–Paleogene sediments in cores FL-533 and FL-422 (Fig. 1). 2. Methods Core Fl-533 recovered 348 cm of sediment. The bottom 67 cm of Core Fl-533 consists of black organic-rich mud, the top 2 cm of which are laminated (Fig. 2). This is overlain by 20 cm of yellowish clayey mud with very low organic content. Above this is 261 cm of late Pliocene to Pleistocene glacialmarine sediment that is correlated with well known Arctic lithostratigraphic units F to M (Clark et al., 1986). One centimeter thick samples were taken at one centimeter intervals throughout the bottom 60 cm of black mud, and at 3 cm intervals through the next 25 cm of black mud and yellowish clayey mud. Samples were processed with HCl and HF and kerogen slides were made using glycerine. An ini-


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Fig. 2. Relative abundances of selected taxa and groups of taxa in Core Fl-533, based on counts of up to 300 specimens per sample. Log10 P=G ratio D Log10 (# peridinioid cysts=# gonyaulacoid cysts). Lithologic section, hydrogen and oxygen indices, percent total organic carbon (TOC), and Ž 13 C values are from Clark et al. (1986).

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tial study of these slides indicated that the overall assemblage of organic-walled phytoplankton cysts was the same throughout the black mud. A subset of the samples was processed further, using heavy liquid separation, oxidation, and acetolysis to remove amorphous organic matter (AOM) and plant debris, so that detailed study of the phytoplankton cysts could be made. Permanently mounted strewn slides were made and examined at magnifications of 250 to 2000ð. Further processing of kerogen residue affected some taxa: Comasphaeridium sp. 1 and Veryhachium sp. 1 were selectively lost or degraded, such that their abundance in the palynologic slides is significantly less than in the kerogen slides. Because of this, numeric counts were not made on the palynologic slides. Counts of up to 300 specimens per sample were made from the kerogen slides, and both the kerogen and palynologic slides were examined completely for the presence of taxa outside of the counted number. Because aggregates of small cysts often occur within clumps of AOM in the kerogen slides, these were excluded from the counts (see further discussion in Section 3). Images were made using a video CCD camera mounted on a Zeiss Axiophot microscope, and connected to an image capture board in a Macintosh computer. Contrast and brightness of the images was adjusted using Adobe Photoshop. Plates I–XI illustrate all taxa recorded in this study and listed in Table 1. 3. Results

and amorphous matter occur at 2.75–2.76 m and 2.78–2.79 m depth in the core, within the yellowish clayey mud. These show a dull orange fluorescence, equivalent to scale point 2a of Tyson (1995). Above 2.75 m, no phytoplankton cysts occur, but sparse plant debris still exists. Clark et al. (1986) described the sedimentology and organic geochemistry of the black mud in this core, and concluded that it was formed by deposition of a large amount of terrestrial organic matter in a marine mud, mixed with abundant marine organic matter (Fig. 2). Palynologic evidence supports a mix of marine and terrestrial organic matter, but perhaps not as much terrestrial organic matter as thought by Clark et al. (1986). Based on geochemistry and carbon isotope data, Clark et al. (1986) also concluded that the laminated part of the black mud and the lower part of the yellowish clayey mud show a decrease in marine organic matter. Palynologic analysis also supports this conclusion. The most abundant palynomorphs are small (5– 25 µm), round, one-walled cysts. Numerically, they overwhelm all other palynomorphs combined, and often permeate large clumps of AOM (Plate IX, 5– 6). They were not included in numeric counts because of the difficulty of counting individual cysts within the large clumps of AOM as well as within dispersed AOM. We estimate this group, as a whole, to be 5–10 times more abundant than all the rest of the palynomorph assemblage. Under high magnification, many different types of small cysts can be distinguished (see Plates VII–XI), and can be

3.1. General description The kerogen contains abundant AOM, which is fluffy and irregular in outline, and which contains small dark particles within the fluffy aggregates. The majority of the AOM displays a light brown fluorescence under incident ultraviolet light, equivalent to scale points 3 to 4 in Tyson’s (1995; table 20.2, p. 347) kerogen preservation scale. Most of the structured and amorphous organic matter in the black mud appears to be of marine origin (Plate IX, 5–6). Neither structured plant debris nor terrestrial pollen or spores are common, which supports an offshore marine origin for the mud. Phytoplankton cysts are common in all of the black mud, from 3.48–2.88 m. Very rare phytoplankton cysts and rare plant debris

Plate I 1–2. Adnatosphaeridium sp. 1, 3.11 m, T33, Ph. 3. Florentinia sp. 1, 3.01 m, T32-2, Ph. 4–5. Hystrichosphaeridium tubiferum, 3.08 m, O38, Ph. 6. Achomosphaera ramulifera, 3.01 m, J33-1, Ph. 7. Spiniferites ramosus, 3.44 m, F38, Ph. 8–9. Impagidinium sp. 1, 3.32 m, K34, Ph. 10–11. Areoligera sp. 1, 2.88 m, J9-3, Ph. The processes in this specimen are crumpled and pressed against the body of the cyst. 12. Spongodinium delitiense, 2.98 m, U10, Ph. All photos taken with either transmitted light (Tr), phase contrast (Ph), interference contrast (IC) or UV autofluorescence (Fl). Scale bars in each image are 50 µm. Taxa names are followed by depth in core (m), England Finder coordinates, and illumination type.


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categorized as either acritarchs or as certain types of prasinophyte algal cysts (Tappan, 1980): Cymatiosphaera spp., Leiosphaeridia spp., Discoidella hannae. Close examination of some small cysts revealed that they are individual cells of Palambages, disconnected from their colonies. These could not be reliably counted when clumped together with other small cysts, so only colonies of Palambages were included in the numeric counts. The rest of the marine phytoplankton cyst assemblage contains 49 taxa (Table 1), and is dominated by Comasphaeridium sp. 1, which consists of 60–80% of the assemblage in every sample (Fig. 2). Also common in all samples are, in order of decreasing abundance, Veryhachium sp. 1, Chatangiella biapertura, Spinidinium clavum, Horologinella apiculata, Chlamydophorella sp. 1, and Elytrocysta sp. 1 (Fig. 2). All other taxa have relative abundances of