Dave Bukry and Harry Dowsett of the U.S.. Geological ... Harry Dowsett is also thanked for performing the .... B.M. and Martinson, D,G., 1989. Late Pliocene vari-.
Marine Micropaleontology, 20 (1992) 23-44
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Elsevier Science Publishers B.V., Amsterdam
Pliocene paleoclimatic interpretation of DSDP Site 580 (NW Pacific) using diatoms John A. Barron U.S. Geological Survey, MS 915, Menlo Park, CA 94025, USA (Received May 22, 1992; revision accepted June 23, 1992 )
ABSTRACT Barron, J.A., 1992. Pliocene paleoclimatic interpretation of DSDP Site 580 (NW Pacific) using diatoms. Mar. Micropaleontol., 20: 23-44. High-resolution quantitative diatom data are tabulated for the early part of the late Pliocene ( 3.25 to 2.08 Ma ) at DSDP Site 580 in the northwestern Pacific. Sample spacing averages 11 k.y. between 3.1 and 2.8 Ma, but increases to 14 to 19 k.y. prior to 3.1 Ma and after 2.8 Ma. Q-mode factor analysis of the middle Pliocene assemblage reveals four factors which explain 92.4% of the total variance of the 47 samples studied between 3.25 and 2.55 Ma. Three of the factors are closely related to modern subarctic, transitional, and subtropical elements, while the fourth factor, which is dominated by Coscinodiscus marginatus and the extinct Pliocene species Neodenticula kamtschatica, appears to correspond to a middle Pliocene precursor of the subarctic water mass. Knowledge of the modern and generalized Pliocene paleoclimatic relationships of various diatom taxa is used to generate a paleoclimate curve ("Twt") based on the ratio of warm-water (subtropical) to cold-water diatoms with warm-water transitional taxa (Thalassionema nitzschioides, Thalassiosira oestrupii, and Coscinodiscus radiatus) factored into the equation at an intermediate (0.5) value. The "Twt" ratios at more southerly DSDP Sites 579 and 578 are consistently higher (warmer) than those at Site 580 throughout the Pliocene, suggesting the validity of the ratio as a paleoclimatic index. Diatom paleoclimatic data reveal a middle Pliocene (3.1 to 3.0 Ma) warm interval at Site 580 during which paleotemperatures may have exceeded maximum Holocene values by 3 °-5.5 °C at least three times. This middle Pliocene warm interval is also recognized by planktic foraminifers in the North Atlantic, and it appears to correspond with generalized depleted oxygen isotope values suggesting polar warming. The diatom "Twt" curve for Site 580 compares fairly well with radiolarian and silicoflagellate paleoclimatic curves for Site 580, planktic foraminiferal sea-surface temperature estimates for the North Atlantic, and benthic oxygen isotope curves for late Pliocene, although higher resolution studies on paired samples are required to test the correspondence of these various paleoclimatic indices.
Introduction As photosynthetic algae that are abundant and diverse in both high- and low-latitude waters, diatoms are very sensitive to surficial water mass conditions. Studies by Kanaya and Koizumi (1966) and Sancetta and Silvestri (1986) demonstrate that diatoms present in Correspondence to: J.A. Barron, U.S. Geological Survey, Mail Stop 915, Menlo Park, CA 94025, USA.
0377-8398/92/$05.00
surficial seafloor sediment in the North Pacific can be successfully used to characterize overlying water masses, even though considerable numbers of finely-silicified diatom taxa living in surface waters may not be preserved in deepsea sediments. Sancetta and Silvestri (1986) tabulated diatom assemblages in 216 core top samples from throughout the North Pacific (north of 30 °N ) and used cluster analysis to produce nine statistically distinct clusters with characteristic
© 1992 Elsevier Science Publishers B.V. All rights reserved.
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taxonomic compositions (Table 1). A subtropical cluster (Cluster A) is associated with the relatively warm, saline and well stratified waters underlying the subtropical gyre of the North Pacific. A second cluster (Cluster I) was recognized along the seaward edge of the California Current and is also characterized by subtropical taxa. North of the subtropical gyre lies a transition zone (the Subarctic Front) which is characterized by steep temperature and salinity gradients, with values decreasing northward. Sancetta and Silvestri's (1986) Cluster B underlies the southern part of the transition zone and consists of a mixture of subtropical and cosmopolitan taxa, while their Cluster C corresponds to the north part of the transition zone and is composed of both subarctic and cosmopolitan diatoms. The cyclonic subarctic gyre to the north of the transition zone is characterized by a salinity minimum and includes Clusters D and E of Sancetta and Silvestri (1986) These clusters are very similar and are composed of mostly subarctic taxa, with assemblages of Cluster D being somewhat more diverse and diluted by taxa from neighboring waters. Cluster F (northwest margin) is also subarctic in character, but it is also composed of neritic and pelagic taxa that are characteristic of highly productive northern latitude waters. According to Sancetta and Silvestri ( 1986 ), the Sea of Okhotsk has a distinctive diatom assemblage that is characterized by high percentages of Thalassiosira trifulta ( = T. latimarginata ) and Actinocyclus curvatulus (Cluster G ). Waters within the Sea of Okhotsk are cold, flesh (32-33%o), and highly stratified compared to those of the subarctic North Pacific. Sancetta and Silvestri ( 1986 ) also recognize a Cluster H centered on the Kuril Islands which appears to be a mixture of the Okhotsk (Cluster G) and northwest margin (Cluster F) assemblages.
J.A. BARRON
TABLE 1
Sancetta and Silvestri's ( 1986 ) North Pacific clusters of diatom assemblages from core-top data. Taxonomy has been updated, and established ancestral forms with similar distribution are shown within parentheses
Cluster
Region
Characteristic diatoms
A
Subtropical
15-20% T. nitzschiodes: 5-
B
C
South Transition
North
Transition
D
Subarctic 1
E
Subarctic 2
F
NW margin
G
Okhotsk
H
Kuril
I
California
10% A. nodulifera, A. tabularis, C. radiatus, P. sulcata, T. oestrupii; < 5% H. cuneiformis, P. doliolus 15-20% T. oestrupii; 5-10% ,4. curvatulus, C. radiatus, P. doliolus (N. fossilis), T. nitzschioides, T. latimarginata 20-30% N. seminae (N. koizumii); 10-20% C. marginatus; 10-15% A. curvatulus: 5-10% T. nitzschioides, T. oestrupii, T. latimarginata 30-40% N. seminae (N. koizumii); 5-10% A. curvatulus, Rz. hebetata, 72 latimarginata: 10% Chaetoceros, Thalasiosira spp. (Bering Sea only) 50-70% N. seminae (N. koizumii); 5-10% Chaetoceros, Rz. hebetata, T. latimarginata 15-30% Chaetoceros; 1020% N. seminae (N. koizumii), T. antarctica; 510% B. fragilis, O. aurita, T. latimarginata, Tx. longissima 25-40% T. latimarginata; 20-35% A. curvatulus; 5-10% B. fragilis, Chaetoceros, N. seminae (N. koizumiO, Tx. Iongissima 20-20% T. latimargmata; 10-20% N. seminae (N. koizumii); 10% A. curvatulus, Chaetoceros 35-40% T. nitzschioides; 515% P. doliolus (N. fossilis), T. oestrupii, 5% C. radiatus
DIATOM DATA OF DSDP SITE 580 (NW PACIFIC)
Paleotemperature studies DSDP Site 580 (41 °37.47'N, 153°58.58'E, water depth 5375 m) in the northwest Pacific just north of the Subarctic Front (Fig. 1 ) has been the focus of numerous detailed paleoclimatic studies using siliceous microfossils. The lack of a carbonate record precluded oxygen isotope studies on foraminifers. At Site 580 an expanded Pliocene and Quaternary section was hydraulically piston-cored to a depth of 154 meters below seafloor (mbsf). Age control for Site 580 is provided by the paleomagnetic stratigraphy of Bleil ( 1985 ) which identifies all of the paleomagnetic event boundaries from the Matuyama Reversed-Polarity Chron-Brunhes Normal-Polarity Chron boundary (0.73 Ma) down to the base of the Mammoth ReversedPolarity Subchron (3.18 Ma) of the Gauss Normal-Polarity Chron near the base of Hole 580 (Table 2 ). At present, Hole 580 is the only hydraulic piston-cored site in the North Pacific lying north of the Subarctic Front. Detailed paleoclimatic studies using diatoms have been completed on DSDP Site 580 by Koizumi (1985), Sancetta (1985), and Sancetta and Silvestri (1986). Sancetta's ( 1985 ) study was limited to the latest Quaternary, while Sancetta and Silvestri (1985) studied samples spaced about every 50 cm down to about 2.6 Ma (late Pliocene). Koizumi (1985) studied the entire Site 580 section at a sampling interval of one sample per section (or every 1.5 m), a time resolution of 30-40 k.y. Sancetta's (1985) study of the Brunhes Normal-Polarity Chron (last 73 k.y. ) record at Site 580 showed that the diatom assemblage varied on a scale that seem to correlate well with glacial-interglacial stages, but the lack of any isotope record for the site precluded any such correlation. She concluded that the flora deposited during the early Brunhes NormalPolarity Chron was characterized by the presence of true subtropical diatoms, the middle Brunhes was marked by high-amplitude fluc-
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tuations in species typical of the transition zone, and the late Brunhes saw an increase in subarctic species. Sancetta and Silvestri (1986) concluded that the present-day Subarctic Gyre was established progressively in the North Pacific during the last 2.4 m.y. through steps of increasing decoupling with the Subtropical Gyre at about 2.4, 1.6-1.5, 1.0, 0.8-0.7, and 0.35 Ma. They provided very detailed downcore percentage data from Sites 579 and 580 and three piston cores in the North Pacific for the last 2.6 m.y. According to Sancetta and Silvestri (1986), prior to 2.5 Ma the modern subarctic water mass did not exist in the western Pacific and a broad transition zone extending from south of 41°N to north of 48°N was present in the North Pacific. They suggested that vertical mixing and productivity in the North Pacific north of 40°N were greater than they are at present and little or no sea ice was produced in marginal seas during the winter. Koizumi ( 1985 ) used a ratio of subtropical (Azpeitia nodulifera and Hemidiscus cuneiformis ) to subarctic ( Denticulopsis seminae ) to calculate a relative diatom paleotemperature curve for Site 580 and Site 579 (38°37.68'N, 153 ° 50.17'E, water depth 5737 m ), which lies south of the present-day Subarctic Front (Fig. 1). Denticulopsis seminae sensu Koizumi (1985) includes both Neodenticula seminae (Simonsen and Kanaya) Akiba and Yanagisawa and N. koizumii Akiba and Yanagisawa based on the taxonomic revision of Akiba and Yanagisawa (1985). Koizumi (1985) recognized large drops in surface water temperature between approximately 2.85 and 2.5 Ma, at 2.0, and at 1.6 Ma. Koizumi ( 1985 ) also identified an interval at about 3.0 Ma that was characterized by surface water paleotemperatures in excess of those values of the present day at the two sites. Morley and Dworetzky ( 1991 ) compared the radiolarian paleoclimatic records of Site 580 with that of Site 579 over the past 3.0 m.y. They concluded that during the late Pliocene
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