D. Krawczyk, A. Witkowski, M. Moros, J. Lloyd, J. Høyer, A. Miettinen, A. Kuijpers
Holocene oceanographic conditions in Disko Bay, West Greenland, were reconstructed from high-resolution diatom records derived from two marine sediment cores. A modern data set composed of 35 dated surface sediment samples collected along the West Greenland coast accompanied by remote sensing data was used to develop a diatom transfer function to reconstruct April sea ice concentration (SIC) supported by July sea surface temperature (SST) in the area. Our quantitative reconstruction shows that oceanographic changes recorded throughout the last ~11,000 years reflect seasonal interplay between spring (April SIC) and summer (July SST) conditions. Our records show clear correlation with climate patterns identified from ice core data from GISP2 and Agassiz-Renland for the early to middle Holocene. The early Holocene deglaciation of western Greenland Ice Sheet was characterized in Disko Bay by initial strong centennial-scale fluctuations in April SIC with amplitude of over 40%, followed by high April SIC and July SST. These conditions correspond to a general warming of the climate in the Northern Hemisphere. A decrease in April SIC and July SST was recorded during the Holocene Thermal Optimum reflecting more stable spring-summer conditions in Disko Bay. During the late Holocene, high April SIC characterized the Medieval Climate Anomaly, while high July SST prevailed during the Little Ice Age, supporting previously identified antiphase relationship between surface waters in West Greenland and climate in NW Europe. This antiphase pattern might reflect seasonal variations in regional oceanographic conditions and large-scale fluctuations within the North Atlantic Oscillation and Atlantic Meridional Overturning Circulation.
{"title":"Quantitative reconstruction of Holocene sea ice and sea surface temperature off West Greenland from the first regional diatom data set","authors":"D. Krawczyk, A. Witkowski, M. Moros, J. Lloyd, J. Høyer, A. Miettinen, A. Kuijpers","doi":"10.1002/2016PA003003","DOIUrl":"https://doi.org/10.1002/2016PA003003","url":null,"abstract":"Holocene oceanographic conditions in Disko Bay, West Greenland, were reconstructed from high-resolution diatom records derived from two marine sediment cores. A modern data set composed of 35 dated surface sediment samples collected along the West Greenland coast accompanied by remote sensing data was used to develop a diatom transfer function to reconstruct April sea ice concentration (SIC) supported by July sea surface temperature (SST) in the area. Our quantitative reconstruction shows that oceanographic changes recorded throughout the last ~11,000 years reflect seasonal interplay between spring (April SIC) and summer (July SST) conditions. Our records show clear correlation with climate patterns identified from ice core data from GISP2 and Agassiz-Renland for the early to middle Holocene. The early Holocene deglaciation of western Greenland Ice Sheet was characterized in Disko Bay by initial strong centennial-scale fluctuations in April SIC with amplitude of over 40%, followed by high April SIC and July SST. These conditions correspond to a general warming of the climate in the Northern Hemisphere. A decrease in April SIC and July SST was recorded during the Holocene Thermal Optimum reflecting more stable spring-summer conditions in Disko Bay. During the late Holocene, high April SIC characterized the Medieval Climate Anomaly, while high July SST prevailed during the Little Ice Age, supporting previously identified antiphase relationship between surface waters in West Greenland and climate in NW Europe. This antiphase pattern might reflect seasonal variations in regional oceanographic conditions and large-scale fluctuations within the North Atlantic Oscillation and Atlantic Meridional Overturning Circulation.","PeriodicalId":19882,"journal":{"name":"Paleoceanography","volume":"32 1","pages":"18-40"},"PeriodicalIF":0.0,"publicationDate":"2017-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1002/2016PA003003","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"51094234","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
L. Max, N. Rippert, L. Lembke‐Jene, A. Mackensen, D. Nürnberg, R. Tiedemann
We provide high-resolution foraminiferal stable carbon isotope (δ13C) records from the subarctic Pacific and Eastern Equatorial Pacific (EEP) to investigate circulation dynamics between the extratropical and tropical North Pacific during the past 60 kyr. We measured the δ13C composition of the epibenthic foraminiferal species Cibicides lobatulus from a shallow sediment core recovered from the western Bering Sea (SO201-2-101KL; 58°52.52′N, 170°41.45′E; 630 m water depth) to reconstruct past ventilation changes close to the source region of Glacial North Pacific Intermediate Water (GNPIW). Information regarding glacial changes in the δ13C of subthermocline water masses in the EEP is derived from the deep-dwelling planktonic foraminifera Globorotaloides hexagonus at ODP Site 1240 (00°01.31′N, 82°27.76′W; 2921 m water depth). Apparent similarities in the long-term evolution of δ13C between GNPIW, intermediate waters in the eastern tropical North Pacific and subthermocline water masses in the EEP suggest the expansion of relatively 13C-depleted, nutrient-enriched, and northern sourced intermediate waters to the equatorial Pacific under glacial conditions. Further, it appears that additional influence of GNPIW to the tropical Pacific is consistent with changes in nutrient distribution and biological productivity in surface waters of the glacial EEP. Our findings highlight potential links between North Pacific mid-depth circulation changes, nutrient cycling, and biological productivity in the equatorial Pacific under glacial boundary conditions.
{"title":"Evidence for enhanced convection of North Pacific Intermediate Water to the low-latitude Pacific under glacial conditions","authors":"L. Max, N. Rippert, L. Lembke‐Jene, A. Mackensen, D. Nürnberg, R. Tiedemann","doi":"10.1002/2016PA002994","DOIUrl":"https://doi.org/10.1002/2016PA002994","url":null,"abstract":"We provide high-resolution foraminiferal stable carbon isotope (δ13C) records from the subarctic Pacific and Eastern Equatorial Pacific (EEP) to investigate circulation dynamics between the extratropical and tropical North Pacific during the past 60 kyr. We measured the δ13C composition of the epibenthic foraminiferal species Cibicides lobatulus from a shallow sediment core recovered from the western Bering Sea (SO201-2-101KL; 58°52.52′N, 170°41.45′E; 630 m water depth) to reconstruct past ventilation changes close to the source region of Glacial North Pacific Intermediate Water (GNPIW). Information regarding glacial changes in the δ13C of subthermocline water masses in the EEP is derived from the deep-dwelling planktonic foraminifera Globorotaloides hexagonus at ODP Site 1240 (00°01.31′N, 82°27.76′W; 2921 m water depth). Apparent similarities in the long-term evolution of δ13C between GNPIW, intermediate waters in the eastern tropical North Pacific and subthermocline water masses in the EEP suggest the expansion of relatively 13C-depleted, nutrient-enriched, and northern sourced intermediate waters to the equatorial Pacific under glacial conditions. Further, it appears that additional influence of GNPIW to the tropical Pacific is consistent with changes in nutrient distribution and biological productivity in surface waters of the glacial EEP. Our findings highlight potential links between North Pacific mid-depth circulation changes, nutrient cycling, and biological productivity in the equatorial Pacific under glacial boundary conditions.","PeriodicalId":19882,"journal":{"name":"Paleoceanography","volume":"32 1","pages":"41-55"},"PeriodicalIF":0.0,"publicationDate":"2017-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1002/2016PA002994","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"51094397","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
L. Menviel, Jimin Yu, F. Joos, A. Mouchet, K. Meissner, M. England
Atmospheric CO₂ was ~90 ppmv lower at the Last Glacial Maximum (LGM) compared to the late Holocene, but the mechanisms responsible for this change remain elusive. Here we employ a carbon isotope-enabled Earth System Model to investigate the role of ocean circulation in setting the LGM oceanic δ¹³C distribution, thereby improving our understanding of glacial/interglacial atmospheric CO₂ variations. We find that the mean ocean δ¹³C change can be explained by a 378 ± 88 Gt C(2σ) smaller LGM terrestrial carbon reservoir compared to the Holocene. Critically, in this model, differences in the oceanic δ¹³C spatial pattern can only be reconciled with a LGM ocean circulation state characterized by a weak (10–15 Sv) and relatively shallow (2000–2500 m) North Atlantic Deep Water cell, reduced Antarctic Bottom Water transport (≤10 Sv globally integrated), and relatively weak (6–8 Sv) and shallow (1000–1500 m) North Pacific Intermediate Water formation. This oceanic circulation state is corroborated by results from the isotope-enabled Bern3D ocean model and further confirmed by high LGM ventilation ages in the deep ocean, particularly in the deep South Atlantic and South Pacific. This suggests a poorly ventilated glacial deep ocean which would have facilitated the sequestration of carbon lost from the terrestrial biosphere and atmosphere.
{"title":"Poorly ventilated deep ocean at the Last Glacial Maximum inferred from carbon isotopes: A data-model comparison study","authors":"L. Menviel, Jimin Yu, F. Joos, A. Mouchet, K. Meissner, M. England","doi":"10.1002/2016PA003024","DOIUrl":"https://doi.org/10.1002/2016PA003024","url":null,"abstract":"Atmospheric CO₂ was ~90 ppmv lower at the Last Glacial Maximum (LGM) compared to the late Holocene, but the mechanisms responsible for this change remain elusive. Here we employ a carbon isotope-enabled Earth System Model to investigate the role of ocean circulation in setting the LGM oceanic δ¹³C distribution, thereby improving our understanding of glacial/interglacial atmospheric CO₂ variations. We find that the mean ocean δ¹³C change can be explained by a 378 ± 88 Gt C(2σ) smaller LGM terrestrial carbon reservoir compared to the Holocene. Critically, in this model, differences in the oceanic δ¹³C spatial pattern can only be reconciled with a LGM ocean circulation state characterized by a weak (10–15 Sv) and relatively shallow (2000–2500 m) North Atlantic Deep Water cell, reduced Antarctic Bottom Water transport (≤10 Sv globally integrated), and relatively weak (6–8 Sv) and shallow (1000–1500 m) North Pacific Intermediate Water formation. This oceanic circulation state is corroborated by results from the isotope-enabled Bern3D ocean model and further confirmed by high LGM ventilation ages in the deep ocean, particularly in the deep South Atlantic and South Pacific. This suggests a poorly ventilated glacial deep ocean which would have facilitated the sequestration of carbon lost from the terrestrial biosphere and atmosphere.","PeriodicalId":19882,"journal":{"name":"Paleoceanography","volume":"32 1","pages":"2-17"},"PeriodicalIF":0.0,"publicationDate":"2017-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1002/2016PA003024","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"51094600","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
M. Makarova, J. Wright, K. Miller, T. Babila, T. Babila, Y. Rosenthal, Jill I. Park
We present new δ13C and δ18O records of surface (Morozovella and Acarinina) and thermocline dwelling (Subbotina) planktonic foraminifera and benthic foraminifera (Gavelinella, Cibicidoides, and Anomalinoides) during the Paleocene-Eocene Thermal Maximum (PETM) from Millville, New Jersey and compare them with three other sites located along a paleoshelf transect from the U.S. mid-Atlantic coastal plain. Our analyses show different isotopic responses during the PETM in surface versus thermocline and benthic species. Whereas all taxa record a 3.6-4.0 ‰ δ13C decrease associated with the carbon isotope excursion (CIE), thermocline dwellers and benthic foraminifera show larger δ18O decreases compared to surface dwellers. We consider two scenarios that can explain the observed isotopic records: 1) a change in the water column structure; and 2) a change in habitat or calcification season of the surface dwellers due to environmental stress (e.g., warming, ocean acidification, surface freshening, and/or eutrophication). In the first scenario, persistent warming during the PETM would have propagated heat into deeper layers and created a more homogenous water column with a thicker warm mixed layer and deeper, more gradual thermocline. We attribute the hydrographic change to decreased meridional thermal gradients, consistent with models that predict polar amplification. The second scenario assumes that environmental change was greater in the mixed layer forcing surface dwellers to descend into thermocline waters as a refuge or restrict their calcification to the colder seasons. Though both scenarios are plausible, similar δ13C responses recorded in surface, thermocline, and benthic foraminifera challenge mixed layer taxa migration.
{"title":"Hydrographic and ecologic implications of foraminiferal stable isotopic response across the U.S. mid-Atlantic continental shelf during the Paleocene-Eocene Thermal Maximum","authors":"M. Makarova, J. Wright, K. Miller, T. Babila, T. Babila, Y. Rosenthal, Jill I. Park","doi":"10.1002/2016PA002985","DOIUrl":"https://doi.org/10.1002/2016PA002985","url":null,"abstract":"We present new δ13C and δ18O records of surface (Morozovella and Acarinina) and thermocline dwelling (Subbotina) planktonic foraminifera and benthic foraminifera (Gavelinella, Cibicidoides, and Anomalinoides) during the Paleocene-Eocene Thermal Maximum (PETM) from Millville, New Jersey and compare them with three other sites located along a paleoshelf transect from the U.S. mid-Atlantic coastal plain. Our analyses show different isotopic responses during the PETM in surface versus thermocline and benthic species. Whereas all taxa record a 3.6-4.0 ‰ δ13C decrease associated with the carbon isotope excursion (CIE), thermocline dwellers and benthic foraminifera show larger δ18O decreases compared to surface dwellers. We consider two scenarios that can explain the observed isotopic records: 1) a change in the water column structure; and 2) a change in habitat or calcification season of the surface dwellers due to environmental stress (e.g., warming, ocean acidification, surface freshening, and/or eutrophication). In the first scenario, persistent warming during the PETM would have propagated heat into deeper layers and created a more homogenous water column with a thicker warm mixed layer and deeper, more gradual thermocline. We attribute the hydrographic change to decreased meridional thermal gradients, consistent with models that predict polar amplification. The second scenario assumes that environmental change was greater in the mixed layer forcing surface dwellers to descend into thermocline waters as a refuge or restrict their calcification to the colder seasons. Though both scenarios are plausible, similar δ13C responses recorded in surface, thermocline, and benthic foraminifera challenge mixed layer taxa migration.","PeriodicalId":19882,"journal":{"name":"Paleoceanography","volume":"32 1","pages":"56-73"},"PeriodicalIF":0.0,"publicationDate":"2017-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1002/2016PA002985","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"51094220","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Based on new U-Th ages of corals drilled offshore Barbados, Abdul et al. (2016) have confirmed the existence of the abrupt stratigraphic feature called meltwater pulse 1B (MWP-1B), which they interpret as being due to a very large and global sea level step change dated at about 11.3 kyr before present (approximately 15 m and equivalent to twice the amount of water stored in the present Greenland ice sheet). This contrasts with the Tahiti record, in which MWP-1B is essentially absent or very small, as Carlson and Clark (2012) and Lambeck et al. (2014) also conclude in their recent reviews of deglacial sea levels at the global scale. However, the evidence provided by Abdul et al. and their main conclusions are not convincing as they are affected by the following three main problems, which may explain the apparent discrepancies: Problem #1/Barbados is located in a subduction zone, which was also active throughout the Late Glacial period. Furthermore, the Barbados cores studied by Abdul et al. were drilled on both sides of the extension of a tectonic feature identified at the southern tip of Barbados (South Point) as underlined by several studies of the Barbados stratigraphy. Problem #2/Fossil samples of Acropora palmata may not be reliable sea level markers during rapid and large sea level rises. Indeed, the asexual reproduction strategy of this species may not be optimal to keep up when the water depth is increasing very rapidly. This may in part explain why the living depth of A. palmata at Barbados was significantly greater than 5 m during some periods of the last deglaciation, notably between 14.5 and 14 kyr B.P. and possibly between 14 and 11.5 kyr B.P. Problem #3/The slow glacio-isostatic adjustment and the rapid responses due to gravitational changes of ice and water masses complicate the interpretation of individual relative sea level (RSL) records at specific locations. Therefore, the Barbados and Tahiti record cannot be compared directly in terms of absolute sea level values as done by Abdul et al. In addition, different glaciohydroisostatic adjustments at the two sites may also have contributed to the observed discrepancy between their deglacial RSL records.
{"title":"Comment on Younger Dryas sea level and meltwater pulse 1B recorded in Barbados reef crest coral Acropora palmata by Abdul et al. (2016 Paleoceanography)","authors":"E. Bard, B. Hamelin, P. Deschamps, G. Camoin","doi":"10.1002/2016PA002979","DOIUrl":"https://doi.org/10.1002/2016PA002979","url":null,"abstract":"Based on new U-Th ages of corals drilled offshore Barbados, Abdul et al. (2016) have confirmed the existence of the abrupt stratigraphic feature called meltwater pulse 1B (MWP-1B), which they interpret as being due to a very large and global sea level step change dated at about 11.3 kyr before present (approximately 15 m and equivalent to twice the amount of water stored in the present Greenland ice sheet). This contrasts with the Tahiti record, in which MWP-1B is essentially absent or very small, as Carlson and Clark (2012) and Lambeck et al. (2014) also conclude in their recent reviews of deglacial sea levels at the global scale. However, the evidence provided by Abdul et al. and their main conclusions are not convincing as they are affected by the following three main problems, which may explain the apparent discrepancies: Problem #1/Barbados is located in a subduction zone, which was also active throughout the Late Glacial period. Furthermore, the Barbados cores studied by Abdul et al. were drilled on both sides of the extension of a tectonic feature identified at the southern tip of Barbados (South Point) as underlined by several studies of the Barbados stratigraphy. Problem #2/Fossil samples of Acropora palmata may not be reliable sea level markers during rapid and large sea level rises. Indeed, the asexual reproduction strategy of this species may not be optimal to keep up when the water depth is increasing very rapidly. This may in part explain why the living depth of A. palmata at Barbados was significantly greater than 5 m during some periods of the last deglaciation, notably between 14.5 and 14 kyr B.P. and possibly between 14 and 11.5 kyr B.P. Problem #3/The slow glacio-isostatic adjustment and the rapid responses due to gravitational changes of ice and water masses complicate the interpretation of individual relative sea level (RSL) records at specific locations. Therefore, the Barbados and Tahiti record cannot be compared directly in terms of absolute sea level values as done by Abdul et al. In addition, different glaciohydroisostatic adjustments at the two sites may also have contributed to the observed discrepancy between their deglacial RSL records.","PeriodicalId":19882,"journal":{"name":"Paleoceanography","volume":"1 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2016-12-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1002/2016PA002979","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"51093637","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Xiaoli Zhou, Ellen Thomas, A. Winguth, A. Ridgwell, H. Scher, B. Hoogakker, R. Rickaby, Zunli Lu
Anthropogenic warming could well drive depletion of oceanic oxygen in the future. Important insight into the relationship between deoxygenation and warming can be gleaned from the geological record, but evidence is limited because few ocean oxygenation records are available for past greenhouse climate conditions. We use I/Ca in benthic foraminifera to reconstruct late Paleocene through early Eocene bottom and pore water redox conditions in the South Atlantic and Southern Indian Oceans and compare our results with those derived from Mn speciation and the Ce anomaly in fish teeth. We conclude that waters with lower oxygen concentrations were widespread at intermediate depths (1.5–2 km), whereas bottom waters were more oxygenated at the deepest site, in the Southeast Atlantic Ocean (>3 km). Epifaunal benthic foraminiferal I/Ca values were higher in the late Paleocene, especially at low-oxygen sites, than at well-oxygenated modern sites, indicating higher seawater total iodine concentrations in the late Paleocene than today. The proxy-based bottom water oxygenation pattern agrees with the site-to-site O2 gradient as simulated in a comprehensive climate model (Community Climate System Model Version 3), but the simulated absolute dissolved O2 values are low (< ~35 µmol/kg), while higher O2 values (~60–100 µmol/kg) were obtained in an Earth system model (Grid ENabled Integrated Earth system model). Multiproxy data together with improvements in boundary conditions and model parameterization are necessary if the details of past oceanographic oxygenation are to be resolved.
{"title":"Expanded oxygen minimum zones during the late Paleocene-early Eocene: Hints from multiproxy comparison and ocean modeling","authors":"Xiaoli Zhou, Ellen Thomas, A. Winguth, A. Ridgwell, H. Scher, B. Hoogakker, R. Rickaby, Zunli Lu","doi":"10.1002/2016PA003020","DOIUrl":"https://doi.org/10.1002/2016PA003020","url":null,"abstract":"Anthropogenic warming could well drive depletion of oceanic oxygen in the future. Important insight into the relationship between deoxygenation and warming can be gleaned from the geological record, but evidence is limited because few ocean oxygenation records are available for past greenhouse climate conditions. We use I/Ca in benthic foraminifera to reconstruct late Paleocene through early Eocene bottom and pore water redox conditions in the South Atlantic and Southern Indian Oceans and compare our results with those derived from Mn speciation and the Ce anomaly in fish teeth. We conclude that waters with lower oxygen concentrations were widespread at intermediate depths (1.5–2 km), whereas bottom waters were more oxygenated at the deepest site, in the Southeast Atlantic Ocean (>3 km). Epifaunal benthic foraminiferal I/Ca values were higher in the late Paleocene, especially at low-oxygen sites, than at well-oxygenated modern sites, indicating higher seawater total iodine concentrations in the late Paleocene than today. The proxy-based bottom water oxygenation pattern agrees with the site-to-site O2 gradient as simulated in a comprehensive climate model (Community Climate System Model Version 3), but the simulated absolute dissolved O2 values are low (< ~35 µmol/kg), while higher O2 values (~60–100 µmol/kg) were obtained in an Earth system model (Grid ENabled Integrated Earth system model). Multiproxy data together with improvements in boundary conditions and model parameterization are necessary if the details of past oceanographic oxygenation are to be resolved.","PeriodicalId":19882,"journal":{"name":"Paleoceanography","volume":"31 1","pages":"1532-1546"},"PeriodicalIF":0.0,"publicationDate":"2016-12-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1002/2016PA003020","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"51094915","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
The recognition of astronomically forced (Milankovitch) climate cycles in geological archives marked a major advance in Earth science, revealing a heartbeat within the climate system of general importance and key utility. Power spectral analysis is the primary tool used to facilitate identification of astronomical cycles in stratigraphic data, but commonly employed methods for testing the statistical significance of relatively high narrow-band variance of potential astronomical origin in spectra have been criticized for inadequately balancing the respective probabilities of type I (false positive) and type II (false negative) errors. This has led to suggestions that the importance of astronomical forcing in Earth history is overstated. It can be readily demonstrated, however, that the imperfect nature of the stratigraphic record and the quasiperiodicity of astronomical cycles sets an upper limit on the attainable significance of astronomical signals. Optimized significance testing is that which minimizes the combined probability of type I and type II errors. Numerical simulations of stratigraphically preserved astronomical signals suggest that optimum significance levels at which to reject a null hypothesis of no astronomical forcing are between 0.01 and 0.001 (i.e., 99–99.9% confidence level). This is lower than commonly employed in the literature (90–99% confidence levels). Nevertheless, in consonance with the emergent view from other scientific disciplines, fixed-value null hypothesis significance testing of power spectra is implicitly ill suited to demonstrating astronomical forcing, and the use of spectral analysis remains a difficult and subjective endeavor in the absence of additional supporting evidence.
{"title":"Optimizing significance testing of astronomical forcing in cyclostratigraphy","authors":"D. Kemp","doi":"10.1002/2016PA002963","DOIUrl":"https://doi.org/10.1002/2016PA002963","url":null,"abstract":"The recognition of astronomically forced (Milankovitch) climate cycles in geological archives marked a major advance in Earth science, revealing a heartbeat within the climate system of general importance and key utility. Power spectral analysis is the primary tool used to facilitate identification of astronomical cycles in stratigraphic data, but commonly employed methods for testing the statistical significance of relatively high narrow-band variance of potential astronomical origin in spectra have been criticized for inadequately balancing the respective probabilities of type I (false positive) and type II (false negative) errors. This has led to suggestions that the importance of astronomical forcing in Earth history is overstated. It can be readily demonstrated, however, that the imperfect nature of the stratigraphic record and the quasiperiodicity of astronomical cycles sets an upper limit on the attainable significance of astronomical signals. Optimized significance testing is that which minimizes the combined probability of type I and type II errors. Numerical simulations of stratigraphically preserved astronomical signals suggest that optimum significance levels at which to reject a null hypothesis of no astronomical forcing are between 0.01 and 0.001 (i.e., 99–99.9% confidence level). This is lower than commonly employed in the literature (90–99% confidence levels). Nevertheless, in consonance with the emergent view from other scientific disciplines, fixed-value null hypothesis significance testing of power spectra is implicitly ill suited to demonstrating astronomical forcing, and the use of spectral analysis remains a difficult and subjective endeavor in the absence of additional supporting evidence.","PeriodicalId":19882,"journal":{"name":"Paleoceanography","volume":"31 1","pages":"1516-1531"},"PeriodicalIF":0.0,"publicationDate":"2016-12-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1002/2016PA002963","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"51093904","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Sea surface temperature (SST) in subtropical eastern boundary upwelling zones is shown to be affected by three main factors: large-scale ocean stratification, upwelling-favorable sea surface wind stress, and the surface concentration (baroclinicity) of the alongshore pressure gradient driving the incoming geostrophic flow which balances the Ekman surface outflow. Pliocene-aged SST proxies suggest that some combination of differences in upwelling forcing enable the sea surface temperatures in these zones to increase by up to 11∘C. We find that large warming in SST in response to the three factors, of up to about 10∘C in addition to a mean Pliocene ocean warming of 2–3∘C, is concentrated in the direct upwelling zone. In the location of proxy sea surface temperatures, about 120 km away from the coast, and outside the coastal upwelling zone, the SST response to changes in wind and stratification is weaker, only accounting for up to 3.4∘C above the mean Pliocene warming. Increased baroclinicity of the alongshore pressure gradient has a smaller effect, accounting for less than 2∘C increases at both the coast and proxy site. The SST seaward (westward) of the upwelling zone is primarily determined by ocean-atmosphere heat exchange and basin-scale ocean forcing, rather than by changes in upwelling. The spatial pattern of SST change with each of the three forcing factors is similar, and therefore, all could contribute to the Pliocene-modern difference in coastal SST.
{"title":"The effect of changes in surface winds and ocean stratification on coastal upwelling and sea surface temperatures in the Pliocene","authors":"Madeline D. Miller, E. Tziperman","doi":"10.1002/2016PA002996","DOIUrl":"https://doi.org/10.1002/2016PA002996","url":null,"abstract":"Sea surface temperature (SST) in subtropical eastern boundary upwelling zones is shown to be affected by three main factors: large-scale ocean stratification, upwelling-favorable sea surface wind stress, and the surface concentration (baroclinicity) of the alongshore pressure gradient driving the incoming geostrophic flow which balances the Ekman surface outflow. Pliocene-aged SST proxies suggest that some combination of differences in upwelling forcing enable the sea surface temperatures in these zones to increase by up to 11∘C. We find that large warming in SST in response to the three factors, of up to about 10∘C in addition to a mean Pliocene ocean warming of 2–3∘C, is concentrated in the direct upwelling zone. In the location of proxy sea surface temperatures, about 120 km away from the coast, and outside the coastal upwelling zone, the SST response to changes in wind and stratification is weaker, only accounting for up to 3.4∘C above the mean Pliocene warming. Increased baroclinicity of the alongshore pressure gradient has a smaller effect, accounting for less than 2∘C increases at both the coast and proxy site. The SST seaward (westward) of the upwelling zone is primarily determined by ocean-atmosphere heat exchange and basin-scale ocean forcing, rather than by changes in upwelling. The spatial pattern of SST change with each of the three forcing factors is similar, and therefore, all could contribute to the Pliocene-modern difference in coastal SST.","PeriodicalId":19882,"journal":{"name":"Paleoceanography","volume":"32 1","pages":"371-383"},"PeriodicalIF":0.0,"publicationDate":"2016-12-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1002/2016PA002996","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"51094520","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Abdul et al. (2016) presented a detailed record of sea level at Barbados (13.9 - 9 kyr B.P.) tightly constraining the timing and amplitude during the Younger Dryas and Meltwater Pulse 1B (MWP-1B) based on U-Th dated reef crest coral species Acropora palmata. The Younger Dryas slow-stand and the large (14 meter) rapid sea-level jump are not resolved in the Tahiti record. Tahiti sea-level estimates are remarkably close to the Barbados sea level curve between 13.9 and 11.6 kyr but fall below the Barbados sea-level curve for a few thousand years following MWP-1B. By 9 kyr the Tahiti sea level estimates again converge with the Barbados sea level curve. Abdul et al. (2016) concluded that Tahiti reefs at the core sites did not keep up with intervals of rapidly rising sea level during MWP-1B. We counter Bard et al. (2016) by showing: 1) there is no evidence for a hypothetical fault in Oistins Bay affecting one of the Barbados coring locations: 2) that the authors confuse the rare occurrences of A. palmata at depths > 5 meters with the “thickets” of A. palmata fronds representing the reef-crest facies, and 3) that uncertainties in depth habitat proxies largely account for differences in Barbados and Tahiti sea-level differences curves with A. palmata providing the most faithful proxy. Given the range in Tahiti paleo-depth uncertainties at the cored sites, the most parsimonious explanation remains that Tahiti coralgal ridges did not keep up with the sea level-rise of MWP-1B.
{"title":"Reply to comment by E. Bard et al. on “Younger Dryas sea level and meltwater pulse 1B recorded in Barbados reef crest coral Acropora palmata” by N. A. Abdul et al.","authors":"R. Mortlock, N. A. Abdul, J. Wright, R. Fairbanks","doi":"10.1002/2016PA003047","DOIUrl":"https://doi.org/10.1002/2016PA003047","url":null,"abstract":"Abdul et al. (2016) presented a detailed record of sea level at Barbados (13.9 - 9 kyr B.P.) tightly constraining the timing and amplitude during the Younger Dryas and Meltwater Pulse 1B (MWP-1B) based on U-Th dated reef crest coral species Acropora palmata. The Younger Dryas slow-stand and the large (14 meter) rapid sea-level jump are not resolved in the Tahiti record. Tahiti sea-level estimates are remarkably close to the Barbados sea level curve between 13.9 and 11.6 kyr but fall below the Barbados sea-level curve for a few thousand years following MWP-1B. By 9 kyr the Tahiti sea level estimates again converge with the Barbados sea level curve. Abdul et al. (2016) concluded that Tahiti reefs at the core sites did not keep up with intervals of rapidly rising sea level during MWP-1B. We counter Bard et al. (2016) by showing: 1) there is no evidence for a hypothetical fault in Oistins Bay affecting one of the Barbados coring locations: 2) that the authors confuse the rare occurrences of A. palmata at depths > 5 meters with the “thickets” of A. palmata fronds representing the reef-crest facies, and 3) that uncertainties in depth habitat proxies largely account for differences in Barbados and Tahiti sea-level differences curves with A. palmata providing the most faithful proxy. Given the range in Tahiti paleo-depth uncertainties at the cored sites, the most parsimonious explanation remains that Tahiti coralgal ridges did not keep up with the sea level-rise of MWP-1B.","PeriodicalId":19882,"journal":{"name":"Paleoceanography","volume":"31 1","pages":"1609-1616"},"PeriodicalIF":0.0,"publicationDate":"2016-12-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1002/2016PA003047","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"51094713","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
The TEX86 and U37K' molecular biomarker proxies have been broadly applied in downcore marine sediments to reconstruct past sea surface temperature (SST). Although both TEX86 and U37K' have been interpreted as proxies for mean annual SST throughout the global ocean, regional studies of glycerol dibiphytanyl glycerol tetraethers (GDGTs) and alkenones in sinking particles are required to understand the influence of seasonality, depth distribution, and diagenesis on downcore variability. We measure GDGT and alkenone flux, as well as the TEX86 and U37K' indices in a 4 year sediment trap time series (2010–2014) in the northern Gulf of Mexico (nGoM), and compare these data with core-top sediments at the same location. GDGT and alkenone fluxes do not show a consistent seasonal cycle; however, the largest flux peaks for both occurs in winter. U37K' covaries with SST over the 4 year sampling interval, but the U37K'-SST relationship in this data set implies a smaller slope or nonlinearity at high temperatures when compared with existing calibrations. Furthermore, the flux-weighted U37K' value from sinking particles is significantly lower than that of underlying core-top sediments, suggesting preferential diagenetic loss of the tri-unsaturated alkenone in sediments. TEX86 does not covary with SST, suggesting production in the subsurface upper water column. The flux-weighted mean TEX86 matches that of core-top sediments, confirming that TEX86 in the nGoM reflects local planktonic production rather than allochthonous or in situ sedimentary production. We explore potential sources of uncertainty in both proxies in the nGoM but demonstrate that they show nearly identical trends in twentieth century SST, despite these factors.
{"title":"GDGT and alkenone flux in the northern Gulf of Mexico: Implications for the TEX86 and UK'37 paleothermometers","authors":"J. Richey, J. Tierney","doi":"10.1002/2016PA003032","DOIUrl":"https://doi.org/10.1002/2016PA003032","url":null,"abstract":"The TEX86 and U37K' molecular biomarker proxies have been broadly applied in downcore marine sediments to reconstruct past sea surface temperature (SST). Although both TEX86 and U37K' have been interpreted as proxies for mean annual SST throughout the global ocean, regional studies of glycerol dibiphytanyl glycerol tetraethers (GDGTs) and alkenones in sinking particles are required to understand the influence of seasonality, depth distribution, and diagenesis on downcore variability. We measure GDGT and alkenone flux, as well as the TEX86 and U37K' indices in a 4 year sediment trap time series (2010–2014) in the northern Gulf of Mexico (nGoM), and compare these data with core-top sediments at the same location. GDGT and alkenone fluxes do not show a consistent seasonal cycle; however, the largest flux peaks for both occurs in winter. U37K' covaries with SST over the 4 year sampling interval, but the U37K'-SST relationship in this data set implies a smaller slope or nonlinearity at high temperatures when compared with existing calibrations. Furthermore, the flux-weighted U37K' value from sinking particles is significantly lower than that of underlying core-top sediments, suggesting preferential diagenetic loss of the tri-unsaturated alkenone in sediments. TEX86 does not covary with SST, suggesting production in the subsurface upper water column. The flux-weighted mean TEX86 matches that of core-top sediments, confirming that TEX86 in the nGoM reflects local planktonic production rather than allochthonous or in situ sedimentary production. We explore potential sources of uncertainty in both proxies in the nGoM but demonstrate that they show nearly identical trends in twentieth century SST, despite these factors.","PeriodicalId":19882,"journal":{"name":"Paleoceanography","volume":"31 1","pages":"1547-1561"},"PeriodicalIF":0.0,"publicationDate":"2016-12-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1002/2016PA003032","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"51094677","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
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