Benthic foraminiferal stable isotope record from ODP Site 138-849 on the East Pacific Rise

Benthic foraminifer and delta13C data from Site 849, on the west flank of the East Pacific Rise (0°11 'N, 110°31'W; 3851 m), give relatively continuous records of deep Pacific Ocean stable isotope variations between 0 and 5 Ma. The mean sample spacing is 4 k.y. Most analyses are from Cibicides wuellerstorfi, but isotopic offsets relative to Uvigerina peregrina appear roughly constant. Because of its location west of the East Pacific Rise, Site 849 yields a suitable record of mean Pacific Ocean delta13C, which approximates a global oceanic signal. The ~100-k.y.-period climate cycle, which is prevalent in delta18O does not dominate the long-term delta13C record. For delta13C, variations in the ~400- and 41-k.y. periods are more important. Phase lags of delta13C relative to ice volume in the 41- and 23-k.y. bands are consistent with delta13C as a measure of organic biomass. A model-calculated exponential response time of 1-2 k.y. is appropriate for carbon stored in soils and shallow sediments responding to glacial-interglacial climate change. Oceanic delta13C leads ice volume slightly in the 100-k.y. band, and this suggests another process such as changes in continental weathering to modulate mean river delta13C at long periods. The delta13C record from Site 849 diverges from that of Site 677 in the Panama Basin mostly because of decay of 13C-depleted organic carbon in the relatively isolated Panama Basin. North Atlantic to Pacific delta13C differences calculated using published data from Sites 607 and 849 reveal variations in Pliocene deep water within the range of those of the late Quaternary. Maximum delta13C contrast between these sites, which presumably reflects maximum influx of high-delta13C northern source water into the deep North Atlantic Ocean, occurred between 1.3 and 2.1 Ma, well after the initiation of Northern Hemisphere glaciation. Export of high-delta13C North Atlantic Deep Water from the Atlantic to the circumpolar Antarctic, as recorded by published delta13C data from Subantarctic Site 704, appears unrelated to the North Atlantic-Pacific delta13C contrast. To account for this observation, we suggest that deep-water formation in the North Atlantic reflects northern source characteristics, whereas export of this water into the circumpolar Antarctic reflects Southern Hemisphere wind forcing. Neither process appears directly linked to ice-volume variations.

Benthic foraminifera in Neogene laminated diatom ooze of ODP Hole 138-844C

Several widely correlatable intervals of laminated Thalassiothrix diatom mat deposits occur in Neogene sediments recovered from the eastern equatorial Pacific Ocean. The presence of laminated sediments in extensive areas of the deep open ocean floor raises fundamental questions concerning the cause of preservation of the laminations and the nature of the benthic environment during episodes of mat deposition. Traditional explanations for the preservation of laminations have centered on restriction of dissolved oxygen. Studies of benthic foraminifers through the laminated intervals show no evidence for an increase in absolute or relative abundance of species characteristic of a low oxygen environment, but rather a decrease in relative abundance of infaunal forms attesting to the impenetrability of the diatom meshwork formed by the interlocking Thalassiothrix frustules. These results support evidence from coring of the high tensile strength of the Thalassiothrix laminations suggesting that the diatom meshwork was of sufficient tensile strength and impenetrability to suppress infaunal benthic activity. Comparison of the relative abundances of foraminifers in the enclosing ôbackgroundö sediment of foraminifer nannofossil ooze and the laminated diatom oozes shows that some epifaunal species (e.g., Cibicides spp.) increase in relative abundance within the laminated sediment, whereas others (e.g., Epistominella exigua) show a marked decrease in relative abundance. Other species show more complex changes in abundance related to the occurrence of the laminated sediments, which may indicate a combination of controls that include the physical nature of the substrate and the amount of organic flux.

Alfredo Cañedo. Aspectos del pensamiento de Leopoldo Lugones. 138 págs. Buenos Aires. Ediciones Marcos, 1974.

Fil: Saguí, Teresita.

Diatom events and Neogene nannofossils of ODP Hole 201-1225A

Shipboard investigation of magnetostratigraphy and shore-based investigation of diatoms and calcareous nannofossils were used to identify datum events in sedimentary successions collected at Ocean Drilling Program (ODP) Leg 201 Site 1225. The goal was to extend the magnetic record previously studied at the same site, ODP Leg 138 Site 851, and provide a comprehensive age model for Site 1225. Two high-magnetic intensity zones at 0-70 and 200-255 meters below seafloor (mbsf) were correlated with lithologic Subunits IA and IC in Hole 1225A. Subunit IA (0-70 mbsf) contains the magnetic reversal record until the Cochiti Subchronozone (3.8 Ma) and has a sedimentation rate of 1.7 cm/k.y. This agrees with previous work done at Site 851. Subunit IC (200-255 mbsf) was not sampled at Site 851. Diatom and nannofossil biostratigraphy constrained this subunit, and we found it to contain the magnetic reversal record between Subchrons C4n.2r and C5n.2n (8.6-9.7 Ma), yielding a sedimentation rate of 2.7 cm/k.y. Biostratigraphy was used to establish the sedimentation rates within Subunits IB and ID (70-200 mbsf and 255-300 mbsf, respectively). These subunits had higher sedimentation rates (~3.4 cm/k.y.) and coincide with the late Miocene-early Pliocene biogenic bloom event (4.5-7 Ma) and the Miocene global cooling trend (10-15 Ma). High biogenic productivity associated with these subunits resulted in the pyritization of the magnetic signal. In lithologic Subunit ID, basement flow is another factor that may be altering the magnetic signal; however, the good correlation between the biostratigraphy and magnetostratigraphy indicates that the magnetic record was locked-in near the seafloor and suggests the age model is robust.