| 1. |
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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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| 11. |
- Mckay, Claire, et al.
(författare)
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The interplay between the surface and bottom water environment within the Benguela Upwelling System over the last 70 ka
- 2016
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Ingår i: Paleoceanography. - 0883-8305. ; 31:2, s. 266-285
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Tidskriftsartikel (refereegranskat)abstract
- The Benguela Upwelling System (BUS), located between 30 and 20°S, is one of the fundamental high-productivity systems of the world ocean. The BUS has previously been studied in terms of primary productivity and ecology over glacial-interglacial timescales; however, the response and coupling with the benthic environment have received little attention. Here, for the first time, we present a high-resolution reconstruction of the BUS highlighting the link between surface and benthic productivity and their response to climatic and oceanographic changes over the last 70 ka. The study is based on benthic foraminiferal faunal analysis together with analyses of diatom assemblages, grain size of the terrigenous fraction, and stable O and C isotopic and bulk biogenic components of core GeoB3606-1. We reveal significant shifts in benthic foraminiferal assemblage composition. Tight coupling existed between the surface and bottom water environment especially throughout marine isotope stages 4 and 3 (MIS4 and MIS3). Due to the high export production, the site has essentially experienced continuous low oxygen conditions; however, there are time periods where the hypoxic conditions were even more notable. Two of these severe hypoxic periods were during parts of MIS4 and MIS3 where we find an inverse relationship between diatom and benthic foraminifera accumulation, meaning that during times of extremely high phytodetritus export we note strongly suppressed benthic productivity. We also stress the importance of food source for the benthos throughout the record. Shifts in export productivity are attributed not only to upwelling intensity and filament front position, but also, regional-global climatic and oceanographic changes had significant impact on the BUS dynamics.
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| 12. |
- Moore, T. C., Jr., et al.
(författare)
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Equatorial Pacific productivity changes near the Eocene-Oligocene boundary
- 2014
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Ingår i: Paleoceanography. - 0883-8305 .- 1944-9186. ; 29:9, s. 825-844
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Tidskriftsartikel (refereegranskat)abstract
- There is general agreement that productivity in high latitudes increased in the late Eocene and remained high in the early Oligocene. Evidence for both increased and decreased productivity across the Eocene-Oligocene transition (EOT) in the tropics has been presented, usually based on only one paleoproductivity proxy and often in sites with incomplete recovery of the EOT itself. A complete record of the Eocene-Oligocene transition was obtained at three drill sites in the eastern equatorial Pacific Ocean (ODP Site 1218 and IODP Sites U1333 and U1334). Four paleoproductivity proxies that have been examined at these sites, together with carbon and oxygen isotope measurements on early Oligocene planktonic foraminifera, give evidence of ecologic and oceanographic change across this climatically important boundary. Export productivity dropped sharply in the basal Oligocene (similar to 33.7Ma) and only recovered several hundred thousand years later; however, overall paleoproductivity in the early Oligocene never reached the average levels found in the late Eocene and in more modern times. Changes in the isotopic gradients between deep- and shallow-living planktonic foraminifera suggest a gradual shoaling of the thermocline through the early Oligocene that, on average, affected accumulation rates of barite, benthic foraminifera, and opal, as well as diatom abundance near 33.5Ma. An interval with abundant large diatoms beginning at 33.3Ma suggests an intermediate thermocline depth, which was followed by further shoaling, a dominance of smaller diatoms, and an increase in average primary productivity as estimated from accumulation rates of benthic foraminifera.
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| 13. |
- Naidu, P. D., et al.
(författare)
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Seasonal sea surface temperature contrast between the Holocene and last glacial period in the western Arabian Sea (Ocean Drilling Project Site 723A): Modulated by monsoon upwelling
- 2005
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Ingår i: Paleoceanography. - 0883-8305. ; 20:1
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Tidskriftsartikel (refereegranskat)abstract
- Annual, summer, and winter sea surface temperatures (SSTs) in the western Arabian Sea were reconstructed through the last 22 kyr using artificial neural networks (ANNs) based on quantitative analyses of planktic foraminifera. Down-core SST estimates reveal that annual, summer, and winter SSTs were 2, 1.2, and 2.6degreesC cooler, respectively, during the last glacial period than in the Holocene. A 2.5degreesC SST increase during Termination 1A (hereinafter referred as glacial to Holocene transition) in the western Arabian Sea. The study reveals a strong seasonal SST contrast between winter and summer from 18 to 14 calendar kyr owing to the combined effect of weak upwelling and strong cold northeasterly winds. Minor or no seasonal SST changes were noticed within the Holocene period, which is attributed to the intense upwelling during the summer monsoon. This causes a lowering of SST to values similar to those of the winter season in analogy with the present day. A 3degreesC rise in winter SSTs during the glacial to Holocene transition coincides with a strengthening of the monsoon, suggesting a link between winter SST and monsoon initiation from the beginning of the Holocene. Strikingly, annual, summer, and winter SSTs show a cooling trend from similar to8 ka to the present day, implying tropical cooling in the late Holocene.
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| 14. |
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| 15. |
- Pearce, Christof, et al.
(författare)
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Heinrich 0 on the east Canadian margin : Source, distribution and timing
- 2015
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Ingår i: Paleoceanography. - 0883-8305 .- 1944-9186. ; 30:12, s. 1613-1624
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Tidskriftsartikel (refereegranskat)abstract
- The last deglaciation was marked by intervals of rapid climatic fluctuations accompanied by glacial advances and retreats along the eastern margin of the Laurentide Ice Sheet (LIS). One of these climatic events, the Younger Dryas cold reversal, was accompanied by a detrital carbonate event referred to as "Heinrich event 0" (H0) that deposited ice-rafted debris and especially detrital carbonate-rich (DC) sediment layers in the western and southern Labrador Sea. The precise age, duration, source, and geographical distribution of the H0 DC event, however, are not entirely clear. A high-sedimentation rate sequence cored off southern Newfoundland yielded an age of ca. 11.5 – 11.3 cal. kyr BP for this layer, thus pointing to its deposition directly following the Younger Dryas termination, likely associated with rapid ice retreat and warming at the onset of the Holocene. At the study site, the H0 layer contains increased concentrations of detrital carbonates (calcite/dolomite ratio ~ 2.5:1) and a mature biomarker composition, similar to that found in Heinrich Layers 1, 2, 4 and 5, which together suggest an origin from Hudson Strait. Grain size analysis indicates that the H0-sediments were transported to the study site mostly through shallow plumes along the Labrador Shelf, with some hyperpycnal dispersal, deeper, along the slope and rise. Our data thus point to a large meltwater release caused by retreat of the ice margin in the Hudson Strait area. The relvatively short duration of the H0 layer, and its timing right after a major climatic transition, combined with its ubiquitous regional occurence makes it valuable for correlating different sediment records from the western Labrador Sea.
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| 16. |
- Plancq, Julien, et al.
(författare)
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Alkenone producers during late Oligocene-early Miocene revisited
- 2012
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Ingår i: Paleoceanography. - 0883-8305 .- 1944-9186. ; 27, s. PA1202-
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Tidskriftsartikel (refereegranskat)abstract
- This study investigates ancient alkenone producers among the late Oligocene–early Miocene coccolithophores recorded at Deep Sea Drilling Project (DSDP) Site 516. Contrary to common assumptions, Reticulofenestra was not the most important alkenone producer throughout the studied time interval. The comparison between coccolith species-specific absolute abundances and alkenone contents in the same sedimentary samples shows thatCyclicargolithus abundances explain 40% of the total variance of alkenone concentration and that the speciesCyclicargolithus floridanus was a major alkenone producer, although other related taxa may have also contributed to the alkenone production at DSDP Site 516. The distribution of the different alkenone isomers (MeC37:2, EtC38:2, and MeC38:2) remained unchanged across distinct changes in species composition, suggesting similar diunsaturated alkenone compositions within the Noelaerhabdaceae family during the late Oligocene–early Miocene. However, the overall larger cell size of Cyclicargolithus may have implications for the alkenone-based reconstruction of past partial pressure of CO2. Our results underscore the importance of a careful evaluation of the most likely alkenone producers for periods (>1.85 Ma) predating the first occurrence of contemporary alkenone producers (i.e., Emiliania huxleyi andGephyrocapsa oceanica).
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| 17. |
- Rasmussen, Tine, et al.
(författare)
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Deep sea records from the southeast Labrador Sea: Ocean circulation changes and ice-rafting events during the last 160,000 years
- 2003
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Ingår i: Paleoceanography. - 0883-8305. ; 18:1
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Tidskriftsartikel (refereegranskat)abstract
- [1] Results from two deep sea cores from northeast of Newfoundland at 1251 and 2527 m water depth, respectively, indicate that during the time period from 160,000 to 10,000 years BP, ice rafting events in the Labrador Sea were accompanied by rapid variations in deep and surface water circulation. Twelve ice-rafting events occurred, each coinciding with high concentrations of detrital carbonate and oxygen isotopic depletion of both surface and bottom waters. Eleven of these can be correlated with the North Atlantic Heinrich events H1-H11. The remaining very conspicuous ice-rafting event took place early in MIS substage 5e, at a time when the planktic faunal assemblage suggests marked warming of the sea surface. In the shallower core, benthic delta(13)C values rise from a minimum during the deglaciation to peak substage 5e values following the last ice-rafting event, indicating that the ventilation of intermediate depths was renewed after the deglaciation was complete and continued throughout substage 5e. The benthic foraminifera suggest that this well-ventilated water mass was comparable to the modern Labrador Sea Water (LSW). The benthic faunas suggest that a relatively warm intermediate water mass entered the SE Labrador Sea during Heinrich events. Generally low benthic delta(13)C values indicate that this water mass was poorly ventilated and rich in inorganic nutrients. Isotope data and benthic faunal distributions indicate that North Atlantic Deep Water (NADW) formed in the Norwegian-Greenland Sea reached the SE Labrador Sea between the Heinrich events.
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| 18. |
- Reghellin, Daniele, et al.
(författare)
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Carbon and oxygen isotopes of bulk carbonate in sediment deposited beneath the eastern equatorial Pacific over the last 8 million years
- 2015
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Ingår i: Paleoceanography. - 0883-8305 .- 1944-9186. ; 30:10, s. 1261-1286
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Tidskriftsartikel (refereegranskat)abstract
- To improve the understanding and utility of bulk carbonate stable carbon and oxygen isotope measurements, we examine sediment from cores in the eastern equatorial Pacific that span the last 8Ma. We measured C-13 and O-18 in 791 samples from Integrated Ocean Drilling Program Site U1338 and Deep Sea Drilling Project Site 573, both located close to the Pacific equator. In 100 samples, we measured C-13 and O-18 on isolated <63 mu m and <38 mu m fractions, which concentrates calcareous nannofossil carbonate and progressively excludes foraminiferal carbonate. Bulk carbonate C-13 and O-18 records are similar to published records from other sites drilled near the equator and seem to reflect mixed layer conditions, albeit with some important caveats involving the precipitation of calcite by coccolithophores. The comparatively lower C-13 and O-18 of the <63 mu m and <38 mu m fractions in sediments younger than 4.4Ma is attributed to an increase in deep-dwelling planktic foraminifera material in bulk carbonate, shifting the bulk isotopic signals toward higher values. Bulk carbonate C-13 is similar over 2500km along the Pacific equator, suggesting covarying concentrations and C-13 of dissolved inorganic carbon within surface waters since 8Ma. Greater bulk sediment C-13 and O-18, higher sedimentation rates, and low content of coarse material suggest intensified wind-driven upwelling and enhanced primary productivity along the Pacific equator between 8.0 and 4.4Ma, although a full understanding of bulk carbonate records will require extensive future work.
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| 19. |
- Schmittner, Andreas, et al.
(författare)
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Calibration of the carbon isotope composition (δ13C) of benthic foraminifera
- 2017
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Ingår i: Paleoceanography. - 0883-8305. ; 32:6, s. 512-530
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Tidskriftsartikel (refereegranskat)abstract
- The carbon isotope composition (δ13C) of seawater provides valuable insight on ocean circulation, air-sea exchange, the biological pump, and the global carbon cycle and is reflected by the δ13C of foraminifera tests. Here more than 1700 δ13C observations of the benthic foraminifera genus Cibicides from late Holocene sediments (δ13CCibnat) are compiled and compared with newly updated estimates of the natural (preindustrial) water column δ13C of dissolved inorganic carbon (δ13CDICnat) as part of the international Ocean Circulation and Carbon Cycling (OC3) project. Using selection criteria based on the spatial distance between samples, we find high correlation between δ13CCibnat and δ13CDICnat, confirming earlier work. Regression analyses indicate significant carbonate ion (-2.6 ± 0.4) × 10-3‰/(μmol kg-1) [CO3 2-] and pressure (-4.9 ± 1.7) × 10-3‰ m-1 (depth) effects, which we use to propose a new global calibration for predicting δ13CDICnat from δ13CCibnat. This calibration is shown to remove some systematic regional biases and decrease errors compared with the one-to-one relationship (δ13CDICnat = δ13CCibnat). However, these effects and the error reductions are relatively small, which suggests that most conclusions from previous studies using a one-to-one relationship remain robust. The remaining standard error of the regression is generally σ ≅ 0.25‰, with larger values found in the southeast Atlantic and Antarctic (σ ≅ 0.4‰) and for species other than Cibicides wuellerstorfi. Discussion of species effects and possible sources of the remaining errors may aid future attempts to improve the use of the benthic δ13C record.
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| 20. |
- Snowball, Ian, et al.
(författare)
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Saw-tooth pattern of North Atlantic current speed during Dansgaard-Oeschger cycles revealed by the magnetic grain size of Reykjanes Ridge sediments at 59 degrees N
- 2003
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Ingår i: Paleoceanography. - 0883-8305. ; 18:2
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Tidskriftsartikel (refereegranskat)abstract
- Detailed mineral (rock) magnetic measurements, including high-resolution magnetic hysteresis loops, were carried out on three sediment cores recovered from the Reykjanes Ridge in the North Atlantic. Supported by physical grain-size analyses, titanomagnetite grain-size variations form a proxy of the speed of near-bottom currents and reveal a cyclic ""saw-tooth'' pattern between Dansgaard-Oeschger cycles 8 and 5. The magnetic grain-size data suggest that these Dansgaard-Oeschger cycles were characterized by gradual intensification of the near-bottom current speed, while other paleoceanographic proxies indicate escalating iceberg discharge and declining sea surface temperatures. These observations are contrary to the frequently assumed and strictly applied positive relationship between high-latitude warmth and North Atlantic Deep Water formation. Within the confines of temporal control, however, it would appear that peaks in iceberg discharge and slow near-bottom current speeds were synchronous. The magnetic hysteresis data also demonstrate that magnetic concentration parameters, such as magnetic susceptibility, do not necessarily provide direct evidence of paleocirculation.
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| 21. |
- Steinke, Stephan, et al.
(författare)
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Reconstructing the southern South China Sea upper water column structure since the Last Glacial Maximum : Implications for the East Asian winter monsoon development
- 2010
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Ingår i: Paleoceanography. - 0883-8305 .- 1944-9186. ; 25, s. PA2219-
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Tidskriftsartikel (refereegranskat)abstract
- Upper water column dynamics in the southern South China Sea were reconstructed in order to track changes in the activity of the East Asian winter monsoon (EAWM) since the Last Glacial Maximum. We used the difference in the stable oxygen isotopes (Δδ18O) and Mg/Ca-based temperatures (ΔT) of surface-dwelling (G. ruber) and thermocline-dwelling (P. obliquiloculata) planktonic foraminifera and the temperature difference between alkenone- and P. obliquiloculata Mg/Ca-based temperatures to estimate the upper ocean thermal gradient at International Marine Past Global Change Study (IMAGES) core MD01-2390. Estimates of the upper ocean thermal gradient were used to reconstruct mixed layer dynamics. We find that our Δδ18O estimates are biased by changes in salinity and, thus, do not display a true upper ocean thermal gradient. The ΔT of G. ruber and P. obliquiloculata as well as the alkenone and P. obliquiloculata suggest increased surface water mixing during the late glacial, likely due to enhanced EAWM winds. Surface water mixing was weaker during the late Holocene, indicating a weaker influence of winter monsoon winds. The weakest winter monsoon activity occurred between 6.5 ka and 2.5 ka. Inferred EAWM changes since the Last Glacial Maximum coincide with EAWM changes as recorded in Chinese loess sediments. We find that the intensity of the EAWM and the East Asian summer monsoon show an inverse behavior during the last glacial and deglaciation but covaried during the middle to late Holocene.
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| 22. |
- Thompson, Bijoy, et al.
(författare)
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Ventilation of the Miocene Arctic Ocean : An idealized model study
- 2010
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Ingår i: Paleoceanography. - 0883-8305 .- 1944-9186. ; 25, s. PA4216-
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Tidskriftsartikel (refereegranskat)abstract
- A model study of an idealized early Miocene Arctic Ocean has been undertaken. The work is motivated by the first drill core retrieved from the Lomonosov Ridge in the central Arctic Ocean, which suggests a transition from anoxic to oxic condition during the early Miocene, a feature presumably related to the opening of the Fram Strait. Here, the ventilation in a semienclosed basin, connected with the ocean through a strait with a sill, is examined using an ocean circulation model that includes a passive age tracer. In particular, we investigate how the ventilation depends on strait geometry, freshwater influx, and surface wind stress. We find that the turnover time, characterizing the bulk ventilation rate, is primarily controlled by the strait width and the wind stress. Generally, the oldest water in the basin is encountered near the sill depth, but wind forcing displaces the oldest water downward. For narrow straits, the turnover time gives an upper bound on the mean age of the basin water. The results have implications when translating local oxygen conditions, recorded in the sediment sequence from the Lomonosov Ridge, to basin-scale circulation patterns. Further, the results indicate that the early Miocene Arctic Ocean became well ventilated when the Fram Strait reached a width of about 100 km.
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| 23. |
- Voigt, Ines, et al.
(författare)
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Holocene shifts of the southern westerlies across the South Atlantic
- 2015
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Ingår i: Paleoceanography. - 0883-8305 .- 1944-9186. ; 30:2, s. 39-51
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Tidskriftsartikel (refereegranskat)abstract
- The southern westerly winds (SWW) exert a crucial influence over the world ocean and climate. Nevertheless, a comprehensive understanding of the Holocene temporal and spatial evolution of the SWW remains a significant challenge due to the sparsity of high-resolution marine archives and appropriate SWW proxies. Here we present a north-south transect of high-resolution planktonic foraminiferal oxygen isotope records from the western South Atlantic. Our proxy records reveal Holocene migrations of the Brazil-Malvinas Confluence (BMC), a highly sensitive feature for changes in the position and strength of the northern portion of the SWW. Through the tight coupling of the BMC position to the large-scale wind field, the records allow a quantitative reconstruction of Holocene latitudinal displacements of the SWW across the South Atlantic. Our data reveal a gradual poleward movement of the SWW by about 1-1.5 degrees from the early to the mid-Holocene. Afterward, variability in the SWW is dominated by millennial scale displacements on the order of 1 degrees in latitude with no recognizable longer-term trend. These findings are confronted with results from a state-of-the-art transient Holocene climate simulation using a comprehensive coupled atmosphere-ocean general circulation model. Proxy-inferred and modeled SWW shifts compare qualitatively, but the model underestimates both orbitally forced multimillennial and internal millennial SWW variability by almost an order of magnitude. The underestimated natural variability implies a substantial uncertainty in model projections of future SWW shifts.
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