Middle to late Miocene fluctuations in the incipient Benguela Upwelling System revealed by calcareous nannofossil assemblages (ODP Site 1085A)

Middle to late Miocene calcareous nannofossil data of ODP Site 1085 from the eastern South Atlantic off Namibia were analysed to document spatial and temporal changes in surface-ocean circulation, upwelling initiation, and associated productivity.

Our data show that calcareous nannofossils constitute a significant part of the carbonate fraction throughout the investigated interval from 12.5 to 7.7 million years (Ma). Highest numbers of calcareous nannofossils (up to 38,000 × 10⁶ nannofossils g^− 1 sediment) were observed during the intervals 9.9 to 9.7 and 8.7 to 8.0 Ma. These elevated numbers of calcareous nannofossils may generally be linked to the initiation of upwelling at about 10 Ma in the studied region. In contrast, diminished numbers of calcareous nannoplankton, as in the interval 9.6 to 9.0 Ma, probably characterise time intervals of weaker productivity resulting in a decrease of nannofossil carbonate contents in the sediments of Site 1085. This decrease in nannofossil production could be one possible explanation for the major CaCO₃ depression in between 9.6 and 9.0 Ma. Coccoliths of the genus Reticulofenestra are the most abundant taxa. Their occurrences are characterised by changes in the investigated time interval. In addition, Coccolithus pelagicus, Calcidiscus leptoporus and Umbilicosphaera spp. contribute a common part of the assemblage. Calcareous nannofossils account for more than half of the carbonate, with peak contribution up to 80% at 8.8 Ma.     

Paleoecological significance of laminated diatomaceous oozes during the middle-to-late Pleistocene,North Atlantic Ocean (IODP Site U1304)

We examined diatom assemblages in a series of remarkable laminated diatomaceous ooze (LDO) horizons in the marine sediments from Integrated Ocean Drilling Program (IODP) Site U1304 to reconstruct the middle-to-late Pleistocene paleoceanographic evolution of the northern North Atlantic Ocean. Four confirmed diatom biohorizons combined with calcareous nannofossil and paleomagnetic stratigraphies established the chronological framework for the material. The planktonic, araphid, needle-like species Thalassiothrix longissima was the greatest contributor to the LDO facies. From the results of a principal component analysis using the percent abundances of 65 significant (p = 5%) diatom taxa, except for Tx. longissima, which was extremely dominant in almost all horizons observed, we identified two principal component (PC) axes. Taxa probably associated with the stratigraphic distribution of the major zonal marker Neodenticula seminae (ranging from 1.26 to 0.84 Ma in this ocean) loaded on PC1 with a high value. PC2 was related to the ocean surface temperature. The stratigraphic variability of the PC2 score indicated that switching between warm- and cold-water assemblages occurred concurrently with LDO deposition (or extreme Tx. longissima dominance) episodes in several horizons (particularly after 0.84 Ma), suggesting that the Subarctic Convergence (SAC) oceanic front passed over Site U1304 during Pleistocene glacial/interglacial cycles. Our floral evidence supports the model of nearly monospecific LDO formation caused by the enhanced physical accumulation of particular diatoms such as Tx. longissima. On the other hand, Nd. seminae, which probably contributes to spring phytoplankton blooms in the modern ocean, was present only between 1.26 and 0.84 Ma in this area. Thus, we infer that the main contributor of export flux in the regional annual primary production cycle would have shifted drastically from one of a spring phytoplankton bloom leader (Nd. seminae) to minor but mass dump assemblages (Tx. longissima etc.) in the mid-Pleistocene.     

Response of calcareous nannofossil assemblages to paleoenvironmental changes through the mid-Pleistocene revolution at Site 1090 (Southern Ocean)

Quantitative study on calcareous nannofossil assemblages has been performed in high time resolution (2–3 kyr) at the Ocean Drilling Program Site 1090. The location of this site in the Southern Ocean is crucial for the comprehension of thermohaline circulation and frontal boundary dynamics, and for testing the employ of nannoflora as paleoceanographical tool. The chronologically well constrained investigated record spans between Marine Isotope Stage (MIS) 35 and 15, through an interval of global paleoclimate and paleoceanographical modification also known as mid-Pleistocene revolution (MPR). Measures of ecological (Shannon–Weaver diversity and paleoproductivity) and dissolution indices together with spectral and wavelet analyses carried out on the acquired time series provide valuable information for interpretation of data in terms of paleoecology and paleoceanography. Assemblages are mainly represented by dominant small Gephyrocapsa, common Calcidiscus leptoporus s.l., Coccolithus pelagicus s.l., Gephyrocapsa (4-5.5 μm), the extinct Pseudoemiliania lacunosa and Reticulofenestra spp. (R. asanoi and Reticulofenestra sp.). Morphotypes discriminated within Calcidiscus leptoporus s.l. and Coccolithus pelagicus s.l., reveal that they may have had different ecological preferences during Pleistocene with respect to the present. The composition and fluctuation in nannofossil assemblage and their comparison with the available Sea Surface Temperature (SST) and C-org curves suggest a primary ecological response to paleoenvironmental changes; relationships to different surface water features and boundary dynamics, as well as to different efficiencies and motions of the intermediate and deep water masses have been inferred. A more northward position of Subantarctic Front (SAF) during most of the Early Pleistocene record has been highlighted based on assemblage composition characterised by common Calcidiscus leptoporus s.l., Coccolithus pelagicus s.l., medium Gephyrocapsa (4–5.5 μm), and by the rarity or absence of Umbilicosphaera spp., Rhabdosphaera spp., Pontosphaera spp., Oolithotus fragilis. Exceptions are the more intense interglacials MIS 31, 17, and probably MIS 15, when a southward displacement of frontal system occurred, coincident with peaks in abundance of Helicosphaera spp. and Syracosphaera spp. Higher nutrient content and more dynamic conditions occurred between MIS 32 and MIS 25, in relation to shallower location of nutrient-rich Antarctic Intermediate Water (AAIW) core and to reduction of glacial–interglacial variability. A nannofossil barren interval is coincident with the known stagnation of South Atlantic deep water circulation during MIS 24, when North Atlantic Deep Water (NADW) was reduced or suppressed and an enhanced northward deep penetration of the more corrosive Circumpolar Deep Water (CPDW) took place. An event of strong instability in nutricline dynamics characterised the transition MIS 23–22 as suggested by sharp fluctuations in paleoproductivity proxies, linked to major changes in oceanographic circulation and to the first distinct increase of larger ice volumes at this time. From MIS 21 upward the nannofossil variations seem to be primarily controlled by glacial–interglacial cyclicity and temperature fluctuations. The cyclic fluctuation recognised in nannofossil abundance seems to be linked to orbitally-forced climatic variation, primarily to the obliquity periodicity recorded in the patterns of C. leptoporus intermediate (5–8 μm) and C. pelagicus pelagicus (6–10 μm); however no obvious and linear relations may be always observed between nannoflora fluctuation and Milankovitch parameters, suggesting more complex and unclear relationships between nannofossils and environmental change.     

The Middle Miocene climatic transition in the Southern Ocean: Evidence of paleoclimatic and hydrographic changes at Kerguelen plateau from planktonic foraminifers and stable isotopes

Middle Miocene (14.8–11.9 Ma) deep-sea sediments from ODP Hole 747A (Kerguelen Plateau, southern Indian Ocean) contain abundant, well-preserved and diverse planktonic foraminiferal assemblages. A detailed study of the climatic and hydrographic changes that occurred in this region during the Middle Miocene Climatic Transition led to the identification of an intense cooling phase (the Middle Miocene Shift). Abundance fluctuations of planktonic foraminiferal species with different paleoclimatic affinities, and oxygen and carbon stable isotopes have been integrated in a multi-proxy approach. Reconstruction of changes in foraminiferal faunal composition and diversity through time were the basis for identification of three foraminiferal biofacies. The most prominent faunal change took place at 13.8 Ma, when a fauna with warm-water affinity (marked by high abundance of Globorotalia miozea group and Globoturborotalita woodi plexus) was replaced by an oligotypic, opportunistic fauna with typical polar characters and dominated by neogloboquadrinids. This faunal change is interpreted as the result of foraminiferal migration from adjacent bioprovinces, caused by modifications in climate and hydrography. A positive 2.0‰ shift in δ¹⁸O (interpreted as the Mi3 event) and a related positive 1.0‰ shift in δ¹³C (corresponding to the CM6 event) accompanied this faunal turnover. These are interpreted to reflect substantial reorganization of Southern Ocean waters, the northward migration of the Polar Front and a strong increase in primary productivity. The second faunal change took place at 12.9 Ma and was characterized by the gradual decrease in abundance of the neogloboquadrinids and the recovery of Globorotalia praescitula/scitula group and Globigerinita glutinata. A positive 1.5‰ shift in δ¹⁸O (interpreted as the Mi4 event) and a concurrent gradual negative shift in δ¹³C accompanied this faunal change, witnessing further modifications of the climate/ocean system. Variations in sea surface temperature, considered as the main factor causing changes of surface hydrography at the Kerguelen Plateau, seem to have been driven by obliquity and long-term eccentricity, thus suggesting a key role played by the astronomical forcing on the evolution of Southern Ocean dynamics during the Middle Miocene. Also an evident 1.2 Myr modulation of the δ¹³C record suggests a main control of the long-term obliquity cycles on the carbon cycle dynamics. Particularly, the Mi3/CM6 events exactly fit with a node of the 1.2 Myr modulation cycles. This confirms the key role played by orbital parameters on high-latitude temperatures and Antarctic ice volume, and indirectly on global carbon burial and/or productivity. This climatic transition was marked also by changes in surface hydrography. From 14.8 to 13.8 Ma an intermediate-strength thermocline controlled by seasonality developed just below the photic zone. Weaker seasonality characterized the interval from 13.8 to 12.9 Ma, when the thermocline became shallower and sharper and favored intermediate-water foraminifers. From 12.9 Ma, seasonality increased again and an intermediate-strength thermocline re-developed.     

Pleistocene fish otoliths from the Mediterranean Basin: a synthesis

An overview of upper pPliocene and pleistocene otolith assemblages is compiled on the basis of both literature data and newly collected material from several sections located mainly in southern Italy. One hundred and five taxa are listed. Additional comments are provided for taxa subject to discussion. The composition and affinities of the Mediterranean Pleistocene otolith associations (consisting mainly of deep sea fishes) is checked against the available data for Pre-Messinian, Pliocene, and Recent Mediterranean fishes. This analysis is based on the recorded nominal species, but some taxa that could be identified at the generic level only are also included when they are relevant from a biogeographic point of view. Some new data on Piacenzian (middle Pliocene) and Gelasian (upper Pliocene) otoliths are also provided because this time interval still constitutes a major gap in the knowledge of the stratigraphic range of Plio-Pleistocene fish taxa. The Pleistocene deepwater fish fauna of the Mediterranean shows a markedly more oceanic character than the present-day one, a tendency that was all ready observed from early Oligocene till Pliocene times. Starting from the late Piacenzian, an increasing number of Atlantic taxa progressively invaded the Mediterranean including several subpolar and temperate forms. This invasion became stronger in the Gelasian. From this period up to the middle and late Pleistocene, the Mediterranean deepwater fish fauna (mesopelagic and benthopelagic) is enriched by taxa which today are typical for the north Atlantic cold deep water. This faunal shift seems to be correlated with the evolution of the Plio-Pleistocene Mediterranean paleoceanographic setting as well as with the global climatic deterioration.     

Mid-Pliocene deep-sea bottom-water temperatures based on ostracode Mg/Ca ratios

We studied magnesium:calcium (Mg/Ca) ratios in shells of the deep-sea ostracode genus Krithe from a short interval in the middle Pliocene between 3.29 and 2.97 Ma using deep-sea drilling sites in the North and South Atlantic in order to estimate bottom water temperatures (BWT) during a period of climatic warmth. Results from DSDP and ODP Sites 552A, 610A, 607, 658A, 659A, 661A and 704 for the period Ma reveal both depth and latitudinal gradients of mean Mg/Ca values. Shallower sites (552A, 610A and 607) have higher mean Mg/Ca ratios (10.3, 9.7, 10.1 mmol/mol) than deeper sites (661A, 6.3 mmol/mol), and high latitude North Atlantic sites (552A, 610A, 607) have higher Mg/Ca ratios than low latitude (658A: 9.8 mmol/mol, 659A: 7.7 mmol/mol, 661A: 6.3 mmol/mol) and Southern Ocean (704: 8.0 mmol/mol) sites. Converting Mg/Ca ratios into estimated temperatures using the calibration of Dwyer et al. (1995) [Dwyer, G.S., Cronin, T.M., Baker, P.A., Raymo, M.E., Buzas, J.S., Corrège, T., 1995. North Atlantic deepwater temperature change during late Pliocene and late Quaternary climatic cycles. Science 270, 1347–1351] suggests that mean middle Pliocene bottom water temperatures at the study sites in the deep Atlantic were about the same as modern temperatures. However, brief pulses of elevated BWT occurred several times between 3.29 and 2.97 Ma in both the North and South Atlantic Ocean suggesting short-term changes in deep ocean circulation.     

Foraminifera and coccolithophorid assemblage changes in the Panama Basin during the last deglaciation: Response to sea-surface productivity induced by a transient climate change

The responses of community assemblages of planktonic and benthonic foraminifera and coccolithophorids to transient climate change are explored for the uppermost 2 m of cores ODP677B (1.2°N; 83.74°W, 3461 m) and TR163-38 (1.34°S; 81.58°W, 2200 m), for the last ∼ 40 ka. Results suggest that the deglaciation interval was a time of increased productivity and a major reorganization of planktonic trophic webs. The succession in dominance between the planktonic foraminifera species Globorotalia inflata, Globigerina bulloides, and Neogloboquadrina pachyderma denote four periods of oceanographic change: (1) advection (24-20 ka), (2) strong upwelling (20-15 ka), (3) weak upwelling (14-8 ka) and (4) oligotrophy (8 ka to present). Strong upwelling for the deglaciation interval is supported by the low Florisphaera profunda/other coccolithophorids ratio and the high percentage abundance of Gephyrocapsa oceanica. Benthonic foraminifera assemblage changes are different in both cores and suggest significant regional variations in surface productivity and/or oxygen content at the seafloor, and a decoupling between surface productivity and export production to the seafloor. This decoupling is evidenced by the inverse relationship between the percentage abundance of infaunal benthonic foraminifera and the percentage abundance of N. pachyderma. The terrigenous input of the Colombian Pacific rivers, particularly the San Juan River, is suggested as a possible mechanism. Finally, the Globorotalia cultrata/Neogloboquadrina dutertrei ratio is used to reconstruct the past influence of the Costa Rica Dome-Panama Bight and cold tongue upwelling systems in the Panama Basin. A northern influence is suggested for the late Holocene (after 5 ka) and the last glacial (before 20 ka), whereas a southern influence is suggested for the 20-5 ka interval. There is a correspondence between our reconstructed northern and southern influences and previously proposed positions of the Intertropical Convergence Zone (ITCZ) after the Last Glacial Maximum (LGM).     

Late Ordovician (Turinian–Chatfieldian) carbon isotope excursions and their stratigraphic and paleoceanographic significance

Five positive carbon isotope excursions are reported from Platteville–Decorah strata in the Upper Mississippi Valley. All occur in subtidal carbonate strata, and are recognized in the Mifflin, Grand Detour, Quimbys Mill, Spechts Ferry, and Guttenberg intervals. The positive carbon isotope excursions are developed in a Platteville–Decorah succession in which background δ¹³C values increase upward from about −2‰ at the base to about 0‰ Vienna Pee Dee belemnite (VPDB) at the top. A regional north–south δ¹³C gradient, with lighter values to the north and heavier values to the south is also noted. Peak excursion δ¹³C values of up to +2.75 are reported from the Quimbys Mill excursion, and up to +2.6 from the Guttenberg excursion, although there are considerable local changes in the magnitudes of these events. The Quimbys Mill, Spechts Ferry, and Guttenberg carbon isotope excursions occur in units that are bounded by submarine disconformities, and completely starve out in deeper, more offshore areas. Closely spaced chemostratigraphic profiles of these sculpted, pyrite-impregnated hardground surfaces show that they are associated with very abrupt centimeter-scale negative δ¹³C shifts of up to several per mil, possibly resulting from the local diagenetic effects of incursions of euxinic bottom waters during marine flooding events.     

The role of the North Atlantic Drift in the millennial timescale glacial climate fluctuations

The Greenland ice core records show that the overall cold climate of the last glacial period was repeatedly interrupted by short, rapid warmings. The events, known as Dansgaard-Oeschger (D-O) events, are strongly imprinted in the North Atlantic marine records suggesting that they were linked to North Atlantic circulation changes. However, the causes of the D-O events are poorly understood and they represent one of the most intriguing puzzles of the last glacial period. In order to investigate a possible mechanism we have studied variations in the distribution of benthic and planktonic foraminifera, oxygen isotopes and ice rafted debris during the last glacial period in eight cores from the North Atlantic and the Nordic Seas taken at mid depth from 853 to 1760 m. The parameters indicate, in agreement with previous studies, that the circulation system during the interstadials resembled the present system. Atlantic surface water flowed north into the Nordic Seas, where most of it sank through convection and, as cold deep water, flowed back into the North Atlantic. During the stadials, a halocline was established in the Nordic Seas and in the northernmost part of the North Atlantic and the outflow from the Nordic Seas stopped. However, below the cold, light surface layer, the relatively warm water of the North Atlantic Drift continued to flow across the North Atlantic and into the Nordic Seas, here warming up the deep-water masses. We suggest that the warm water gradually made the water column unstable. The pivotal abrupt climate warmings were caused by sudden upwelling of the deep, warm water masses causing overturning of the entire water column and onset of convection.     

Late Neogene planktonic foraminifera of the Cibao Valley (northern Dominican Republic): Biostratigraphy and paleoceanography

An assemblage of planktonic foraminifera is described from 125 samples taken from the Cercado, Gurabo, and Mao Formations in the Cibao Valley, northern Dominican Republic. The primary objectives of this study are to establish a biochronologic model for the late Neogene of the Dominican Republic and to examine sea surface conditions within the Cibao Basin during this interval. The Cercado Formation is loosely confined to Zones N17 and N18 ( 7.0–5.9 Ma). The Gurabo Formation spans Zones N18 and N19 ( 5.9–4.5 Ma). The Mao Formation is placed in Zone N19 ( 4.5–3.6 Ma). Changes in the relative abundances of indicator species are used to reconstruct sea surface conditions within the basin. Increasing relative abundances of Globigerinoides sacculifer and Globigerinoides ruber, in conjunction with a decreasing relative abundance of Globigerina bulloides, suggests the onset of increasing sea surface temperature and salinity in conjunction with diminishing primary productivity at 6.0 Ma. Abrupt increases in the relative abundances of G. sacculifer and G. ruber at 4.8 Ma suggest a major increase in sea surface temperature and salinity in the early Pliocene. The most likely mechanism for these changes is isolation of the Caribbean Ocean through progressive restriction of Pacific–Caribbean transfer via the Central American Seaway. Periods of high productivity associated with upwelling events are recorded in the upper Cercado Formation ( 6.1 Ma) and in the middle Mao Formation ( 4.2 Ma) by spikes in G. bulloides and Neogloboquadrina spp. respectively. The timing of major increases in sea surface salinity and temperature as well as decreasing productivity ( 4.8 Ma) and periods of upwelling ( 6.1and 4.2 Ma) in the Cibao Basin generally corroborate previously suggested Caribbean oceanographic changes related to the uplift of Panama. Changes in sea surface conditions depicted by paleobiogeographic distributions in the Cibao Basin suggest that shoaling along the Isthmus of Panama had implications in a shallow Caribbean basin as early as 6.0 Ma. Major paleobiologic changes between 4.8 and 4.2 Ma likely represent the period of final closure of the CAS and a nearly complete disconnection between Pacific and Caribbean water masses. This study illustrates the use of planktonic foraminifera in establishing some paleoceanographic conditions (salinity, temperature, productivity, and upwelling) within a shallow water basin, outlining the connection between regional and localized oceanographic changes.