Marine Micropaleontology 3 1 ( 1997) 177-l 82
ELSEWER
A rapidly deposited pennate diatom ooze in Upper Miocene-Lower Pliocene sediment beneath the North Pacific polar front Gerald R. Dickens a,*,1,John A. Barron b ’ Department of Earth Sciences, James Cook Universit): Townsville, Qld. 4811, Australia ’ United States Geological Survey, Menlo Park, CA 94025. USA
Received 15 September 1996; revised version received 25 November
1996; accepted
5 December
1996
Abstract Rapidly deposited Thalassionema-Thalassiothrti pennate diatom oozes previously have been described in Upper Miocene-Lower Pliocene sediment beneath the frontal boundary of the eastern equatorial Pacific. Here we document a new occurrence of Thalassionema-Thalassiothrix ooze in Upper Miocene-Lower Pliocene sediment beneath the frontal boundary of the subarctic North Pacific. The ooze is a 6 m interval of siliceous sediment at Ocean Drilling Program (ODP) sites 885/886 that was rapidly deposited between approximately 5.0 and 5.9 Ma. Bulk sediment in this interval may contain greater than 85% pennate diatom tests. There are also abundant laminae and pockets that are composed entirely of Thalassionema and Thalassiothrix diatoms. The presence of a rapidly deposited ooze dominated by pennate diatoms indicates unusual past conditions in the overlying surface waters. Time coincident deposition of such oozes at two distinct frontal boundary locations of the Pacific suggests that the unusual surface water conditions were causally linked to large-scale oceanographic change. This same oceanographic change most likely involved (1) addition of nutrients to the ocean, or (2) redistribution of nutrients within the ocean. The occurrence and origin of pennate diatom oozes may be a key component to an integrative understanding of late Neogene paleoceanography and biogeochemical cycling. Keywords:
productivity; Pacific Ocean; biogeochemical cycles; diatoms
1. Introduction A recent and fascinating discovery in micropaleontology has been the documentation of laminated diatom mats in Neogene sediment beneath the eastern equatorial Pacific divergence zone (Fig. 1; Kemp and Baldauf, 1993; Kemp, 1995; Kemp et al., 1995; Pearce et al., 1995). These mats are composed almost exclusively of Thalassiothrix and Thalas* Corresponding author. E-mail:
[email protected] ’ Also at: Department of Geological Sciences, University Michigan, Ann Arbor, MI 48109, USA.
of
0377.8398/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved PII SO377-8398(97)00003-O
sionema pennate diatoms (Baldauf, pers. commun.), and were deposited much faster than sediment sections without mats (Kemp and Baldauf, 1993; Kemp et al., 1995). The occurrence of pennate diatom mats in the eastern Equatorial Pacific is restricted to certain time intervals of the Miocene and Early Pliocene (Kemp and Baldauf, 1993; Kemp, 1995; Kemp et al., 1995). Over the last 10 million years, these time intervals were between approximately 4.3 and 4.5 Ma, 5.1 and 5.8 Ma, and 6.1 and 6.3 Ma according to a modified time scale of Berggren et al. (1985) (Kemp and Baldauf, 1993; Kemp et al., 1995), or 4.6 and 4.9
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.95% silica (by weight, Snoeckx et al., 1995) and was deposited between approximately 5.0 and 5.9 Ma according to the SHCK95 time scale (Dickens et al., 1995; Dickens and Owen, 1996). Thus, the reduced section of silica ooze at sites 885/886 is time coincident with a major episode of mat deposition in the eastern equatorial Pacific (Fig. 2). Samples from Hole 886B were examined for the relative abundance of diatoms to test whether the section of rapid sedimentation at sites 885/886 is also dominated by Thalassiothrix and Thalassionema species. These samples were approximately 1 cm3 aliquots that were taken from 5 cm depth increments of sediment that had been previously homogenized. Counts of > 100 diatom specimens were made for each of these samples (Table 1).
Tabie I Relative abundance of pennate diatoms Thalas.sronemu spp. and Thalmsiothrix spp.in Upper Miocene-Lower Pliocene bulk sediment at ODP Hole 886B Core. section, interval (cm)
Depth
Thalassionenu~
(mcd)
(70’0)
Thafassiothri.r (%)
4H-6. 5- 10 4H-6. SO-85 5H-I. 5-10 5H- I. 80-85 5H-2, 5-10 5H-2. SO-85 SH-3.5-10 5H-3. SO-85 5H-4, 5-10
28.80 29.55 30.05 30.80 31.55 32.30 33.05 33.80 34.55
IO 16 60 82 78 74 63 49 I
3 12 16 5 1 3 7 5 6
The relative abundance of Thulassiothrix and Thalassionema species increases markedly across the reduced section of sediment at sites 885/886 (Fig. 2). These pennate species dominate (>50%) the overall diatom assemblage between approximately 30 and 34 mcd, and reach a peak abundance (87%) at approximately 3 1 med. At the beginning and end of the reduced section, the relative abundance of Thalassiothrix and Thalnssionema species is < 13Yr.
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scale, and over short depth intervals, can approach 100%. Whether the rapidly deposited Thalassiothrix and Thalassionema ooze at sites 8851886 is directly analogous to the ooze observed in Upper MioceneLower Pliocene sediment of the eastern equatorial Pacific is unclear. That is, it cannot be determined whether the majority of this ooze was deposited as discrete mats and subsequently bioturbated (see Kemp, 1995), or alternatively, deposited as individual specimens (or colonies of specimens) from a surficial assemblage that was dominated by pennate diatoms. The Thalassiothrix and Thalassionema laminae and pockets suggest, however, that at least a portion of the ooze at sites 885/886 was deposited in masses (see Kemp, 1995). 3. Significance of latest Miocene-Early pennate diatom oozes
Fig. 3. Scanning electron microscope (SEM) photograph of a stark white ‘spicule ooze’ pocket in Sample 886B-5H-2, 5-10 cm. The pocket is composed of nearly 100% pennate diatoms (in this case, Thalussiorhrix spp.)
Stark white submillimeter laminae and pockets of ‘spicule ooze’ occur in Upper Miocene-Lower Pliocene sediment at sites 885/886 (Rea et al., 1993a). These laminae and pockets are subtle because their color is similar to that of surrounding sediment. However, use of a reflected light microscope reveals their high abundance in samples between 30 and 34 med. Samples of Iaminae and pockets of spicule ooze were removed from unhomogenized sediment from Hole 886B. These samples then were mounted on carbon wafers and placed under a scanning electron microscope (SEM) to test whether the laminae and pockets are composed of Thalussiothrix and ThaZussionema diatoms rather than spicules. Indeed, the ‘spicule ooze’ intervals are almost entirely composed of pennate diatoms (Fig. 3). Thus, the relative abundance of Thalassiothrix and Thalassionema diatoms in Upper Miocene-Lower Pliocene sediment at sites 8851886 is heterogeneous at the millimeter
Pliocene
The presence of a rapidly deposited ooze composed entirely of Thalassiothrix and Thalassionema diatoms requires unusual past surface water conditions that caused these particular pennate diatoms to become extremely abundant at the exclusion of all other test forming species. Two general explanations can be offered. Surface water physics were such that pennate diatoms were concentrated; or, surface water properties (e.g., temperature, chemistry) were such that pennate diatoms proliferated. Kemp and Baldauf (1993) and Kemp et al. (1995) suggested a principally physical mechanism for the origin of rapidly deposited pennate diatom oozes. Pennate diatom colonies in cold surface water subduct beneath warm surface water at a frontal boundary. Because the colonies are buoyant, they return to the surface on the warm side of the frontal boundary. Intensification of the frontal system thus leads to concentration of pennate diatom colonies in a restricted surface area. Concentrated colonies then deplete the nutrients in the surface water, die, and sink in masses. As noted by Kemp et al. (1995), no direct information supports this mechanism. However, it is consistent with all observations (e.g., rapid deposition, assemblage composition), and explains the location of pennate diatom oozes beneath frontal boundaries (Kemp and Baldauf, 1993; Kemp et al., 1995; Pearce et al., 1995). Moreover, the mechanism
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has a present-day analog (Kemp et al., 1995; Pearce et al., 1995): Rhizosolenia diatom mats concentrate on the warm side of the equatorial frontal boundary and sink in masses during intensification of the frontal system. The latest Miocene-Early Pliocene Thdassionema-Thalassiothrix ooze at sites 8851886 presumably reflects the same unusual past surface water conditions that led to deposition of latest MioceneEarly Pliocene Thalassionema-Thalassiothrix mats in the eastern equatorial Pacific. However, sites 885/886 are located greater than 10.000 km from the equatorial frontal boundary. Thus, time coincident deposition of these oozes in two distinct areas suggests some large-scale process caused similar variations in surface water of both regions. Because sites 8851886 are located beneath a major frontal boundary, if the depositional mechanism proposed by Kemp and Baldauf (1993) and Kemp et al. (1995) is correct, then this large-scale process caused intensification of frontal boundaries throughout the subtropical gyre system. The obvious process is a change in ocean/atmosphere dynamics (as also suggested by Kemp et al., 1995). Previous authors (e.g., Keller and Barron, 1987) inferred from widespread lithologic variations in marine sediment cores that a change in ocean/atmosphere dynamics indeed occurred during the latest Miocene-Early Pliocene. Results of two recent studies are of particular interest because they highlight this change in ocean/atmosphere conditions. Eolian accumulation and grain size records in sediment sequences near the equator indicate that trade winds were more intense during the latest Miocene-Early Pliocene than at present-day (Hovan, 1995). Reconstructions of Neogene deep water circulation patterns show that the flux of northern component Atlantic deep water to deep ocean circulation was highest during the latest Miocene-Early Pliocene (Wright and Miller, 1996). Evidence for a change in ocean/atmosphere dynamics at Pacific frontal boundaries during the latest Miocene-Early Pliocene is important to current literature concerning Neogene global geochemical cycling. Numerous authors (e.g., Van Andel et al., 1975; Peterson et al., 1992; Berger et al., 1993; Rea et al., 1993b; Dickens and Owen, 1994, 1996; Delaney and Filippelli, 1994; Farrell et al., 1995a)
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suggested that surticial productivity was significantly higher at divergence zones of the Indian and Pacific oceans between approximately 9.0 and 3.5 Ma on the SHCK95 time scale. Because productivity in the oceans is nutrient limited, and because the residence time of nutrients (e.g., PO:-, NO;, H$SiOJ) in the oceans is relatively short (< lo5 yr), elevated productivity (relative to present-day) at multiple locations over 5.5 million years implies that (1) a process (i.e., enhanced continental weathering) added nutrients to the ocean (Filippelli and Delaney, 1994; Farrell et al., 1995b), or (2) a change in ocean/atmosphere dynamics intensified the flux of nutrients from other regions to divergence zones (Dickens and Owen, 1996). The pennate diatom oozes were deposited during the middle of this 5.5 million year episode of enhanced productivity, and in those regions with compelling evidence for high productivity (e.g., Rea et al., 1993b; Delaney and Filippelli, 1994; Farrell et al., 1995a; Dickens and Owen, 1996). Although the latest Miocene-Early Pliocene high productivity at Indo-Pacific divergence zones may have been enhanced by increased continental weathering, there “likely” was also a contemporaneous change in ocean/atmosphere dynamics that caused intensification of frontal boundaries at Pacific divergence zones and probably increased divergence as well. Acknowledgements
We sincerely thank T.C. Moore Jr. for discussions during this work and J.H. Lipps for a thoughtful review. Funding for G. Dickens was supported by the U.S. Department of Energy under appointment to Graduate Fellowships for Global Change. The scanning electron microscope used in this work was acquired under grant #BSR-83-14092 from the National Science Foundation. Samples for this work were kindly provided by Leg 145 of the Ocean Drilling Program. References Arnold, E., Leinen, M. and King, J., 1995. Paleoenvironmental variation based on the mineralogy and rock-magnetic properties of sediment from Sites 885 and 886. Proc. ODP, Sci. Results, 145: 231-245. Berger, W.H., Leckie, R.M., Janecek, T.R., Stax, R. and Takayama, T., 1993. Neogene carbonate sedimentation on On-
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