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| 2. |
- Beltran, Catherine, et al.
(författare)
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Paleoenvironmental conditions for the development of calcareous nannofossil acme during the late Miocene in the eastern equatorial Pacific
- 2014
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Ingår i: Paleoceanography. - 0883-8305 .- 1944-9186. ; 29:3, s. 210-222
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Tidskriftsartikel (refereegranskat)abstract
- Repeated monospecific coccolithophore dominance intervals (acmes) of specimens belonging to the Noelaerhabdaceae familyincluding the genus Reticulofenestra and modern descendants Emiliania and Gephyrocapsaoccurred during the Neogene. Such acme was recognized during the late Miocene (similar to 8.6Ma), at a time of a major reorganization of nannofossil assemblages resulting in a worldwide temporary disappearance of larger forms of the genus Reticulofenestra (R. pseudoumbilicus) and the gradual recovery and dominance of its smaller forms (< 5 mu m). In this study we present a multiproxy investigation of late Miocene sediments from the east equatorial Pacific Integrated Ocean Drilling Program Site U1338 where small reticulofenestrid-type placoliths with a closed central areaknown as small Dictyococcites spp. (< 3 mu m)formed an acme. We report on oxygen and carbon stable isotope records of multispecies planktic calcite and alkenone-derived sea surface temperature. Our data indicate that, during this 100 kyr long acme, the east equatorial Pacific thermocline remained deep and stable. Local surface stratification state fails to explain this acme and thus contradicts the model-based hypothesis of a Southern Ocean high-latitude nutrient control of the surface waters in the east equatorial Pacific. Instead, our findings suggest that external forcing such as an extended period of low eccentricity may have created favorable conditions for the small Dictyococcites spp. growth. Key Points < list list-type=bulleted id=palo20081-list-0001> < list-item id=palo20081-li-0001> EEP thermocline deep during the late Miocene small Dictyococcites acme Low eccentricity favorable for the small Dictyococcites spp. growth
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| 3. |
- Coxall, Helen K., et al.
(författare)
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Early Oligocene glaciation and productivity in the eastern equatorial Pacific : Insights into global carbon cycling
- 2011
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Ingår i: Paleoceanography. - 0883-8305 .- 1944-9186. ; 26, s. PA2221-
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Tidskriftsartikel (refereegranskat)abstract
- The onset of sustained Antarctic glaciation across the Eocene-Oligocene transition (EOT) marks a pivotal change in Earth's climate, but our understanding of this event, particularly the role of the carbon cycle, is limited. To help address this gap we present the following paleoceanographic proxy records from Ocean Drilling Program Site 1218 in the eastern equatorial Pacific (EEP): (1) stable isotope (delta(18)O and delta(13)C) records generated in epifaunal benthic foraminifera (Cibicidoides spp.) to improve (double the resolution) the previously published records; (2) delta(18)O and delta(13)C records measured on Oridorsalis umbonatus, a shallow infaunal species; and (3) a record of benthic foraminifera accumulation rate (BFAR). Our new isotope data sets confirm the existence at Site 1218 of a two-step delta(18)O increase. They also lend support to the hypothesized existence of a late Eocene transient delta(18)O increase and early Oligocene Oi-1a and Oi-1b glacial maxima. Our record of BFAR indicates a transient (similar to 500 kyr) twofold to threefold peak relative to baseline Oligocene values associated with the onset of Antarctic glaciation that we attribute to enhanced biological export production in the EEP. This takes the same general form as the history of opal accumulation in the Southern Ocean, suggesting strong high-to-low-latitude oceanic coupling. These findings appear to lend support to the idea that the EOT delta(13)C excursion is traceable to increased organic carbon (C(org)) burial. Paradoxically, early Oligocene sediments in the EEP are extremely C(org)-poor, and proxy records of atmospheric pCO(2) indicate a transient increase associated with the EOT.
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| 4. |
- Fontorbe, Guillaume, et al.
(författare)
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Enrichment of dissolved silica in the deep equatorial Pacific during the Eocene-Oligocene
- 2017
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Ingår i: Paleoceanography. - 0883-8305 .- 1944-9186. ; 32, s. 848-863
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Tidskriftsartikel (refereegranskat)abstract
- Silicon isotope ratios (expressed as δ30Si) in marine microfossils can provide insights into silica cycling over geologic time. Here we used δ30Si of sponge spicules and radiolarian tests from the Paleogene Equatorial Transect (Ocean Drilling Program Leg 199) spanning the Eocene and Oligocene (~50–23 Ma) to reconstruct dissolved silica (DSi) concentrations in deep waters and to examine upper ocean δ30Si. The δ30Si values range from 3.16 to +0.18‰ and from 0.07 to +1.42‰ for the sponge and radiolarian records, respectively. Both records show a transition toward lower δ30Si values around 37 Ma. The shift in radiolarian δ30Si is interpreted as a consequence of changes in the δ30Si of source DSi to the region. The decrease in sponge δ30Si is interpreted as a transition from low DSi concentrations to higher DSi concentrations, most likely related to the shift toward a solely Southern Ocean source of deep water in the Pacific during the Paleogene that has been suggested by results from paleoceanographic tracers such as neodymium and carbon isotopes. Sponge δ30Si provides relatively direct information about the nutrient content of deep water and is a useful complement to other tracers of deep water circulation in the oceans of the past.
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| 5. |
- Henderiks, Jorijntje, et al.
(författare)
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Refining ancient carbon dioxide estimates: Significance of coccolithophore cell size for alkenone-based pCO2 records
- 2007
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Ingår i: Paleoceanography. - 0883-8305. ; 22:3, s. PA3202-
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Tidskriftsartikel (refereegranskat)abstract
- Long-term alkenone-based pCO2 records are widely applied in paleo-climate evaluations. These pCO2 estimates are based on records of the carbon isotope fractionation that occurs during marine haptophyte photosynthesis (εp37:2). In addition to the concentration of aqueous CO2 (CO2(aq)), the magnitude of εp37:2 is also influenced by algal growth rates and cell geometry. To date, the influence of haptophyte cell geometry on the expression of ancient εp37:2 values has received little attention.This study evaluates changes in cell geometry of ancient alkenone-producing algae at Deep Sea Drilling Program Site 516 in the southwest Atlantic Ocean by analyzing individual coccolith dimensions, which are proportional to algal cell volume and surface area. We show that during part of the early Miocene, mean cell sizes of alkenone-producing algae were smaller relative to modern E. huxleyi. Cell size variations coincide with significant changes in εp37:2, with a distinct 6‰ decrease in εp37:2 at ~20.3 Ma associated with a 27% increase in haptophyte cell sizes. These changes in cell size impact εp37:2-based interpretations of growth rate variation and CO2(aq) estimates for this southwest Atlantic site. After correcting for cell geometry, CO2(aq) estimates at Site 516 are consistent with those reported from other oligotrophic sites during this time, resulting in overall low atmospheric pCO2 estimates (<350 ppmv) for the early Miocene.
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| 6. |
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| 7. |
- Komar, N., et al.
(författare)
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Understanding long-term carbon cycle trends : The late Paleocene through the early Eocene
- 2013
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Ingår i: Paleoceanography. - : American Geophysical Union (AGU). - 0883-8305 .- 1944-9186. ; 28:4, s. 650-662
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Tidskriftsartikel (refereegranskat)abstract
- The late Paleocene to the early Eocene (∼58–52 Ma) was marked by significant changes in global climate and carbon cycling. The evidence for these changes includes stable isotope records that reveal prominent decreases in δ18O and δ13C, suggesting a rise in Earth's surface temperature (∼4°C) and a drop in net carbon output from the ocean and atmosphere. Concurrently, deep-sea carbonate records at several sites indicate a deepening of the calcite compensation depth (CCD). Here we investigate possible causes (e.g., increased volcanic degassing or decreased net organic burial) for these observations, but from a new perspective. The basic model employed is a modified version of GEOCARB III. However, we have coupled this well-known geochemical model to LOSCAR (Long-term Ocean-atmosphere Sediment CArbon cycle Reservoir model), which enables simulation of seawater carbonate chemistry, the CCD, and ocean δ13C. We have also added a capacitor, in this case represented by gas hydrates, that can store and release13C-depleted carbon to and from the shallow geosphere over millions of years. We further consider accurate input data (e.g., δ13C of carbonate) on a currently accepted timescale that spans an interval much longer than the perturbation. Several different scenarios are investigated with the goal of consistency amongst inferred changes in temperature, the CCD, and surface ocean and deep ocean δ13C. The results strongly suggest that a decrease in net organic carbon burial drove carbon cycle changes during the late Paleocene and early Eocene, although an increase in volcanic activity might have contributed. Importantly, a drop in net organic carbon burial may represent increased oxidation of previously deposited organic carbon, such as stored in peat or gas hydrates. The model successfully recreates trends in Earth surface warming, as inferred from δ18O records, the CCD, and δ13C. At the moment, however, our coupled modeling effort cannot reproduce the magnitude of change in all these records collectively. Similar problems have arisen in simulations of short-term hyperthermal events during the early Paleogene (Paleocene-Eocene Thermal Maximum), suggesting one or more basic issues with data interpretation or geochemical modeling remain.
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| 8. |
- Lear, Caroline H., et al.
(författare)
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Neogene ice volume and ocean temperatures : Insights from infaunal foraminiferal Mg/Ca paleothermometry
- 2015
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Ingår i: Paleoceanography. - 0883-8305 .- 1944-9186. ; 30:11, s. 1437-1454
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Tidskriftsartikel (refereegranskat)abstract
- Antarctic continental-scale glaciation is generally assumed to have initiated at the Eocene-Oligocene Transition, yet its subsequent evolution is poorly constrained. We reconstruct changes in bottom water temperature and global ice volume from 0 to 17 Ma using delta O-18 in conjunction with Mg/Ca records of the infaunal benthic foraminifer, O. umbonatus from Ocean Drilling Program (ODP) Site 806 (equatorial Pacific; similar to 2500m). Considering uncertainties in core top calibrations and sensitivity to seawater Mg/Ca (Mg/Ca)(sw), we produce a range of Mg/Ca-temperature-Mg/Ca-sw calibrations. Our favored exponential temperature calibration is Mg/Ca = 0.66 +/- 0.08 x Mg/Ca-sw(0.27 +/- 0.06) x e((0.114 +/- 0.02 x BWT)) and our favored linear temperature calibration is Mg/Ca = (1.21 +/- 0.04 + 0.12 +/- 0.004 x BWT (bottom water temperature)) x (Mg/Ca-sw -0.003 +/- 0.02) (stated errors are 2 s.e.). The equations are obtained by comparing O. umbonatus Mg/Ca for a Paleocene-Eocene section from Ocean Drilling Program (ODP) Site 690 (Weddell Sea) to delta O-18 temperatures, calculated assuming ice-free conditions during this peak warmth period of the Cenozoic. This procedure suggests negligible effect of Mg/Ca-sw on the Mg distribution coefficient (D-Mg). Application of the new equations to the Site 806 record leads to the suggestion that global ice volume was greater than today after the Middle Miocene Climate Transition (similar to 14Ma). ODP Site 806 bottom waters cooled and freshened as the Pacific zonal sea surface temperature gradient increased, and climate cooled through the Pliocene, prior to the Plio-Pleistocene glaciation of the Northern Hemisphere. The records indicate a decoupling of deep water temperatures and global ice volume, demonstrating the importance of thresholds in the evolution of the Antarctic ice sheet.
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| 9. |
- Lougheed, Bryan C., et al.
(författare)
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Bulk sediment 14C dating in an estuarine environment: : How accurate can it be?
- 2017
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Ingår i: Paleoceanography. - : AMER GEOPHYSICAL UNION. - 0883-8305 .- 1944-9186. ; 32:2, s. 123-131
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Tidskriftsartikel (refereegranskat)abstract
- Abstract Due to a lack of marine macrofossils in many sediment cores from the estuarine Baltic Sea, researchers are often forced to carry out 14C determinations on bulk sediment samples. However, ambiguity surrounding the carbon source pathways that contribute to bulk sediment formation introduces a large uncertainty into 14C geochronologies based on such samples, and such uncertainty may not have been fully considered in previous Baltic Sea studies. We quantify this uncertainty by analyzing bulk sediment 14C determinations carried out on densely spaced intervals in independently dated late-Holocene sediment sequences from two central Baltic Sea cores. Our results show a difference of ~600?14C?yr in median bulk sediment reservoir age, or R(t)bulk, between the two core locations (~1200?14C?yr for one core, ~620?14C?yr for the other), indicating large spatial variation. Furthermore, we also find large downcore (i.e., temporal) R(t)bulk variation of at least ~200?14C?yr for both cores. We also find a difference of 585?14C?yr between two samples taken from the same core depth. We propose that studies using bulk sediment 14C dating in large brackish water bodies should take such spatiotemporal variation in R(t)bulk into account when assessing uncertainties, thus leading to a larger, but more accurate, calibrated age range.
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| 10. |
- Malmgren, BA, et al.
(författare)
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Application of artificial neural networks to chemostratigraphy
- 1996
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Ingår i: PALEOCEANOGRAPHY. - : AMER GEOPHYSICAL UNION. - 0883-8305. ; 11:4, s. 505-512
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Tidskriftsartikel (refereegranskat)abstract
- Artificial neural networks, a branch of artificial intelligence, are computer systems formed by a number of simple, highly interconnected processing units that have the ability to learn a set of target vectors from a set of associated input signals. Neura
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