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| JOURNAL HOME | HELP | CONTACT PUBLISHER | SUBSCRIBE | ARCHIVE | SEARCH | TABLE OF CONTENTS |
,1
1 Department of Earth Sciences, University College London, Gower Street, London WC1E 6BT, England, UK
2 School of Earth, Ocean and Planetary Sciences, Cardiff University, Main Building, Park Place, Cardiff CF10 3YE, Wales, UK
3 Department of Palaeontology, The Natural History Museum, London SW7 5BD, England, UK
4 Department of Geology, Trinity College, Dublin 2, Ireland
5 Tanzania Petroleum Development Corporation, PO Box 2774, Dar-es-Salaam, Tanzania
6 Department of Geology and Geophysics, Texas A&M University, College Station, Texas 77843-3115, USA
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONGEOLOGICAL SETTINGMETHODSKILWA GROUP MICROFOSSILSDISCUSSIONCONCLUSIONSREFERENCES CITED |
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Key Words: calcareous nannofossils • foraminifera • preservation • Lagerstätte • Paleogene • diversity
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGMETHODSKILWA GROUP MICROFOSSILSDISCUSSIONCONCLUSIONSREFERENCES CITED |
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Recent attempts to seek out exceptional foraminifera for geochemical paleoclimate studies have targeted clay-rich, hemipelagic sediments (Wilson et al., 2002; Bice et al., 2003; Pearson et al., 2007). However, the paleobiological potential of these predominantly shelf successions that host well-preserved microfossils remains largely unexploited. To a large extent, this is the result of the enormous amount of stratigraphic and paleoceanographic work that has accompanied the Deep Sea Drilling Project and Ocean Drilling Program since the late 1960s, and the rather uniform state of preservation that is typically associated with such deep-sea chalks and oozes. Cenozoic nannofossil study in particular saw a slowing of taxonomic description after the switch from largely continental-shelf research to deep-sea studies, but the effect is less pronounced in Mesozoic research, which has continued to rely on hemipelagic successions.
The aim of this paper is to exemplify exceptional calcareous microfossil preservation through a description of the Paleogene Kilwa Group of Tanzania. These sediments represent a Konservat-Lagerstätte for calcareous microfossils and provide a benchmark against which to highlight the significant effects that preservation can have on both microfossil diversity and geochemistry.
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GEOLOGICAL SETTING |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGMETHODSKILWA GROUP MICROFOSSILSDISCUSSIONCONCLUSIONSREFERENCES CITED |
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METHODS |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGMETHODSKILWA GROUP MICROFOSSILSDISCUSSIONCONCLUSIONSREFERENCES CITED |
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KILWA GROUP MICROFOSSILS |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGMETHODSKILWA GROUP MICROFOSSILSDISCUSSIONCONCLUSIONSREFERENCES CITED |
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Other common, small coccoliths include minuscule (<1-µm) spinose forms that are, as yet, undescribed (Fig. 3E). They are not easily classified in existing fossil groups but are comparable to the extant Papposphaeraceae and "narrow-rimmed muroliths" (Young et al., 2003, p. 78), which have no previously documented fossil record. Larger coccoliths with fragile central-area structures include new taxa that are difficult to place within existing fossil classifications (Figs. 2I and 3P). Well-known species with delicate structures that have not been previously observed (Figs. 2F, 2J, and 3M) are also preserved. Coccolithus pelagicus specimens, for example, are frequently seen with gracile, axial cross bars (Fig. 3M), demonstrating a subtle morphological difference compared to modern populations, where single transverse bars are common (Young et al., 2003). Delicate central grills are occasionally reported in other coccolith groups, but are routinely observed in the Tanzania material, most notably in Cyclicargolithus, Reticulofenestra, Chiasmolithus, and Cruciplacolithus (Figs. 2F and 3I).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGMETHODSKILWA GROUP MICROFOSSILSDISCUSSIONCONCLUSIONSREFERENCES CITED |
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Holococcoliths and Rhabdoliths
Holococcoliths and small rhabdoliths are not routinely preserved in modern deep-sea sediments (Roth and Berger, 1975), and they are considered to be the most prone to dissolution (Roth and Thierstein, 1972). Holococcoliths are constructed from minute, equidimensional calcite crystallites and formed during the haploid phase of the haplo-diplontic coccolithophore life cycle. They typically alternate with a diploid phase that produces the more commonly observed and robustly constructed heterococcoliths (compare Figs. 2G and 2H). Over 90 holococcolith morphologies have been documented in the modern ocean (32% of the total morphological diversity), but all of them are small (<3 µm) and none typically preserve in the sedimentary record (Young et al., 2003). Living holococcolith-bearing coccolithophores are widely distributed (Kleijne, 1991) and show no particular affinity for shelf environments. Their absence from seafloor sediments is simply the result of a preservational filter, which removes these highly dissolution-prone coccoliths. There are several larger, extinct holococcoliths (e.g., Zygrhablithus; Fig. 3H) that do consistently preserve in the fossil record, but even these are absent in sediments deposited in deeper waters (
3000 m Bown, 2005b). In general, fossil holococcolith preservation is patchy and largely restricted to hemipelagic, clay-rich lithologies. For the Paleogene and Late Cretaceous time intervals, the new taxa described from the Kilwa Group have effectively doubled the known holococcolith fossil diversity, with the addition of 25 Paleogene and 23 Late Cretaceous species (Bown, 2005a; Bown and Dunkley Jones, 2006; Lees, 2007).
Rhabdoliths are also widely distributed in the modern ocean (e.g., Boeckel et al., 2006) but have a patchy and inconsistent fossil record. They are typically spinose and can be large, but minimal dissolution leads to fragmentation of the coccoliths and disarticulation of the spines. Again, their abundance and diversity in the Kilwa Group sediments is the result of exceptional preservation, as can be seen in the delicate rim and spine structures shown in Figures 3N and 3O.
Gladiolithus
Gladiolithus is the most surprising component of the new diversity preserved in the Kilwa Group Figs. (3A–D). It is one of a small number of living coccolithophores specifically adapted to life in the deep photic zone (100–200 m), and it produces highly modified coccoliths and coccospheres that are suspected to be morphological adaptations related to the low-light conditions (Young, 1994). Despite being abundant in the water column, the liths are rarely found at the seafloor (Roth and Berger, 1975), and they have not been previously documented in sediments older than late Quaternary (Okada and Matsuoka, 1996). In fact, there has been no unequivocal documentation of deep-dwelling nannoplankton prior to the Neogene, and of the modern assemblage, only Florisphaera has a fossil record, stretching back to the late Miocene (Young, 1998). We have not observed Florisphaera in the Kilwa Group, suggesting that it evolved in the late Oligocene or Miocene.
The presence of abundant Gladiolithus is significant, not only because it confirms the unique quality of preservation in the Kilwa Group but also because it indicates that a deep–photic-zone niche was exploited by the same group at least back to the late Paleocene (56 Ma). It also lends strong support to the interpretation of these depositional environments as being open ocean and deep water. Modern Gladiolithus are uncommon in shelf seas, and the deep–photic-zone community abundance is strongly correlated with water depth and excluded from marginal basins (Okada, 1983).
Small Taxa, Delicate Structures, and Coccospheres
The occurrence of small coccoliths and fragile, central-area structures, such as those seen in Calciosolenia and Syracosphaera (Figs. 3F and 3K), represents preservation that resembles well-preserved modern coccolithophore material. The presence of coccospheres provides additional paleobiological information that is lost when coccoliths are disaggregated. Generally, only placolith coccoliths that mechanically interlock are found preserved as intact coccospheres in the fossil record (Figs. 2I, 3I, and 3L), while all other fossil taxa are virtually unknown in this state. The Kilwa Group has yielded the only Cenozoic examples of undisturbed, collapsed coccospheres of non-placolith taxa, providing indications of original cell size, coccolith production per cell, and ranges of intraspecific morphological variability (Figs. 3A, 3C, 3E, 3J, and 3K).
Preservation of the Kilwa Group Microfossils
Preservation of the principal calcareous microfossil groups (planktonic foraminifera, benthic foraminifera, and calcareous nanno-plankton) can be affected by the initial degree of shell calcification and postmortem taphonomic and diagenetic processes, including bioturbation, erosion, dissolution, recrystallization, and overgrowth. The planktonic groups live high in the water column (0–200 m) and are exported to the seafloor by simple sinking, in the case of foraminifera (Berger, 1971), or within marine snow aggregates and zooplanktonic fecal pellets, in the case of the smaller nannoplankton (Steinmetz, 1994). Much is known about the dissolution of planktonic foraminifera as they approach the lysocline and sink beneath the calcite compensation depth (Thunell and Honjo, 1981; Schmuker and Schiebel, 2002), but in shallower settings, the death assemblages of planktonic foraminifera are relatively faithful recorders of the overlying living plankton (Bé, 1977). However, in the modern ocean, calcareous nannoplankton are subject to far stronger taphonomic biases that significantly reduce the exported and preserved diversity. This bias is highly correlated with coccolith size, and there appears to be a threshold in preservation potential at 3 µm: 90% of the species with coccoliths >3 µm are found as fossils, compared with only 20% of those with coccoliths <3 µm (Young et al., 2005). This is not direct size selection, but rather the result of small coccoliths having higher surface-area-to-volume ratios, which increases their vulnerability to dissolution. Sediment trap and seafloor samples show that the loss of small coccoliths takes place largely in the water column and is the result of grazing and/or dissolution while sinking, even well above the lysocline (Roth, 1994; Andruleit et al., 2004). Further selective dissolution and fragmentation occurs within the sediment through ingestion by sediment grazers and early diagenesis. With burial, diagenetic processes continue, and it is commonplace to observe deterioration of preservation with increasing depth in deep-sea cores. In carbonate-rich oozes, this involves increase in crystal size at the micron scale, with small crystals being selectively dissolved and larger ones overgrown (Wise, 1977). The net result of the various processes occurring in the water column, at the sediment surface, and during burial, is that even soft oozes are increasingly dominated by larger coccoliths. When the modern, global nannoplankton diversity is compared with the Holocene fossil record, the estimated preserved diversity is, at best, 54% but more typically around 30%. Preserved diversity is even less, if holococcolith morphologies are considered (20%–36%) (Young et al., 2003, 2005). These are significantly high diversity losses that have serious implications for paleontological studies.
The exceptional preservation of calcareous nannofossils in the Paleogene Kilwa Group has resulted in assemblages that contain extraordinarily high species richness, comprising new diversity in well-known families, alongside preservation of small and delicate forms for which we have had no previous fossil record. The majority of this enhanced diversity is explained by preservation rather than paleoecology, and demonstrates the significant effect of favorable taphonomic conditions. The same sediments host planktonic foraminifera assemblages that are not exceptionally diverse but which yield stable isotope values that are considered relatively unaffected by diagenesis. The glassy foraminifera tests, absence of infilling, and primary wall fabrics contrast with deep-sea ooze taphonomy, which is characterized by frosty or white and chalky shells that are considered to result from recrystallization that includes both replacement and overgrowth/infilling (Pearson et al., 2001; Sexton et al., 2006). Post-depositional recrystallization arguably shifts the isotopic values toward early diagenetic calcite and inferred seafloor-environment values that are both colder, in terms of estimated paleotemperatures, and more homogeneous (Pearson et al., 2001, 2007). The remarkable preservation of the calcareous nannofossils strongly corroborates this interpretation of minimal diagenetic modification. The contrasting diversity records of the two microfossil groups, however, highlights the greater sensitivity to preservational modification shown by the smaller-sized nannoplankton.
The quality of preservation is best explained by the clay-rich lithologies that have not been deeply buried. The clays isolate the calcite microfossils tests within an impermeable medium, preventing or inhibiting diagenetic recrystallization. This explanation is supported by excellent organic biomarker preservation that indicates thermal immaturity and low sediment permeability, which has inhibited organic matter biodegradation (van Dongen et al., 2006). There is some variability in the preservation, from sample to sample and even across single SEM samples at the micron scale. Most likely, this reflects heterogeneous microenvironments within the sedimentary fabric, controlled by variations in grain size, porosity, permeability, and sediment chemistry.
Quantifying the Effects of Calcareous Nannofossil Preservation
Although the potential for preservational modification of microfossil assemblages is universally acknowledged, the documentation of preservation is inconsistent. Two main approaches have been used to record microfossil preservation—first, qualitative, visual observations, and second, indices based on indirect evidence, such as fragmentation, dissolution-susceptibility rankings, and abundance comparisons (Berger, 1968; Roth and Thierstein, 1972; Roth and Krumbach, 1986; Le and Shackleton, 1992; Boeckel et al., 2006). More recently, geochemical comparisons between different preservational classes of planktonic foraminifera have been attempted (Sexton et al., 2006). Visual assessment is largely subjective and greatly influenced by worker experience. Dissolution indices have been successful in foraminiferal studies but are not universally applied, and are rarely used in nannofossil studies (Matsuoka, 1990; Gibbs et al., 2004). Furthermore, both approaches may still fail to discriminate cryptic preservational effects that nevertheless significantly alter both the taxonomic and geochemical composition of a microfossil assemblage (Gibbs et al., 2004; Williams et al., 2005; Pearson et al., 2007).
To assess the preserved diversity of Paleogene nannofossils, we have used the Kilwa Group data as a benchmark against which to compare recorded diversities from coeval sections, representing a range of preservation states, for three time slices (Table 1). It is striking that the global compilation returns lower diversities than the Kilwa Group (70%–85%), but the values are broadly comparable, given the uncertainties associated with composite literature surveys (Bown et al., 2004). The individual sections yield diversity values that are, in all cases, considerably less (9%–68%) than the Kilwa Group for each of the three time slices, but there are systematic discrepancies corresponding to section type. The Paleogene shelf sites, with reportedly good preservation (Clayton core—Bybell and Self-Trail, 1995; Bass River—Gibbs et al., 2006; Yazoo Clay, Gulf Coast—Siesser, 1983), host diversities ranging from 43%–68% and compare most favorably with the Kilwa Group values. Deep-sea sections, where carbonate-rich oozes and chalks dominate, yield values ranging from 21%–58%, while the lowest values (9%–24%) come from lithified deepwater limestone sequences (Contessa, Italy). These sections are not all directly comparable, in particular those from higher latitudes (e.g., the Southern Ocean); however, the Eocene was a time of relatively low nannoplankton biogeographic differentiation, and its effects do not greatly bias the data (e.g., only nine species were absent from the Kilwa Group succession due to biogeography). Indeed, the Southern Ocean sites return relatively high diversity values, most likely reflecting better preservation in more clay-rich lithologies.
The negative aspects of carbonate-rich lithologies on the taphonomy of calcareous nannofossils are reasonably well known. However, there has been no serious attempt to quantify these effects, and the degree of taxonomic modification, highlighted in this paper, is probably greater than is generally perceived. This does not usually impact on the stratigraphic application of the group, which generally utilizes taxa selected for size and robustness, but it does have serious implications when assemblage abundance and diversity data are considered. That the least favorable diversity comparisons come from the carbonate-rich successions of tropical and subtropical latitudes is comparable to the morphological- and geochemical-based taphonomic observations from the study of planktonic foraminifera (Sexton et al., 2006).
The potential for large-magnitude nannofossil diversity loss is a significant factor for those using fossil data in paleobiological or paleoceanographic interpretation. These losses can be significantly large when comparing living and Holocene assemblages, as highlighted earlier, and are highly variable in the Paleogene comparisons, described above. However, as in all considerations of the fossil record, there are important caveats to these data that must be considered before such information is dismissed as potentially fallacious, and in the case of the calcareous nannofossils, we are convinced that these explanations justify the long-established value of these paleontological data.
First, the size-range distribution of living coccoliths is strongly skewed toward small sizes (<3 µm; Fig. 5), but this may be an anomalous situation, having followed the sequential evolutionary loss of large taxa through the Pliocene and Pleistocene (Gibbs et al., 2005; Schmidt et al., 2006). Qualitative reviews of pre-Neogene coccolith size suggest that far higher proportions of the total diversity were concentrated in the larger size ranges for much of the last 200 m.y., and this would have significantly increased the proportion of taxa with fossilization potential. The Kilwa Group coccolith-size data support this view and show a very different distribution spectrum to that of modern taxa, with a broader range of sizes, a significantly higher mean length value (8.5 µm versus 3 µm), and higher frequencies throughout the larger size-classes (i.e., >10 µm). Young et al. (2005) argued that subtle changes in coccolith size-frequency through time could result in significant changes in observed diversity, independent of any change in actual diversity, especially if large numbers of coccoliths shifted above or below the 3-µm preservation-potential threshold (Fig. 5). Although we cannot unequivocally prove the fidelity of the Kilwa Group Paleogene fossil record in the <3-µm size range, the observation of abundant coccoliths of this size (e.g., Gladiolithus), with only limited diversity, is strongly suggestive that the skewing seen in the modern group was not as significant in the Paleogene coccolith record.
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Given these critical caveats, we have reason for confidence in the documented fossil record of coccolithophores. Preservation potential may well have been far greater for much of the pre-Quaternary time interval, when coccolith sizes were not as strongly skewed toward the smaller size frequencies, and, excepting the small and fragile coccoliths of the Syracosphaerales, many of the extant groups have good preservation potential, as evidenced by their long and abundant fossil records. However, there remains much to learn about the preservation of coccoliths, and fine-fraction carbonate in general, and a need to develop protocols that allow for the adequate description, quantification, and communication of this essential information. These issues are being addressed in the foraminifera and geochemistry communities, which, by and large, accept that qualitative, descriptive methods of conveying preservation quality are no longer adequate (Pearson et al., 2001; Sexton et al., 2006). Instead, strict criteria that require high-resolution morphological analysis, or indirect geochemical methods, are being used to ensure effective documentation of preservation.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGMETHODSKILWA GROUP MICROFOSSILSDISCUSSIONCONCLUSIONSREFERENCES CITED |
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For both foraminifera and coccolithophores, incorporation into impermeable, clay-rich sediments that have never been deeply buried appears to have been critical in producing the exceptional preservation. The enhanced diversity seen in the calcareous nannofossils highlights the different sensitivities of these two fossil groups to preservational modification. The integrated taphonomic observations from both fossil groups, however, provide the maximum amount of information in support of the interpretation of both geochemical and paleontological proxies.
The Kilwa Group calcareous nannofossil diversities are consistently higher than all coeval assemblages, and even slightly higher than composite global estimates. These comparisons demonstrate the degree of taxonomic modification that can result from varying preservation states. The highest diversities are preserved in hemipelagic, clay-rich lithologies and the greatest losses occur in lithified, carbonate-rich sediments. The majority of the lost diversity, however, is confined to distinct taxonomic groups, and especially the holococcoliths and rhabdoliths (Syracosphaerales). The preservational potential of Paleogene coccolithophores may well have been significantly greater than in the modern oceans because a larger proportion of the biodiversity fell within the larger size fractions.
Study of the Kilwa Group hemipelagic sediments has highlighted the significant effects that preservation can have on both the diversity and geochemistry of calcareous microfossils. These exceptionally preserved fossils are providing high-quality paleontological and geochemical paleoclimate proxy information, and, for the calcareous nannoplankton, this includes paleobiological and biodiversity data that are currently unique for this fossil group.
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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p.bown{at}ucl.ac.uk 
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RECEIVED FOR PUBLICATION 11 May 2007
REVISED MANUSCRIPT RECEIVED 13 August 2007
MANUSCRIPT ACCEPTED 24 August 2007
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