Posts Tagged 'paleo'

Extinction, dissolution, and possible ocean acidification prior to the Cretaceous/Paleogene (K/Pg) boundary in the tropical Pacific

Biotic perturbations and changes in ocean circulation during the Maastrichtian stage of the latest Cretaceous raise questions about whether the biosphere was preconditioned for the end-Cretaceous mass extinction of calcareous plankton. A brief acme of inoceramid clams at ~ 71 Ma on Shatsky Rise in the tropical North Pacific was followed by their extinction during the “mid-Maastrichtian event” at 70.1 Ma associated with an abrupt warming of deep waters. This was later followed by an interval of intense dissolution beginning ~ 67.8 Ma at ODP Site 1209 (2387 m). The late Maastrichtian dissolution interval was initially gradual, and is characterized by a low planktic/benthic (P/B) ratio, highly fragmented planktic foraminifera, mostly an absence of larger taxa, low abundances of smaller taxa, extremely low planktic foraminiferal numbers, and low planktic foraminiferal and nannofossil species richness. A partial recovery in carbonate preservation and calcareous plankton simple diversity began ~ 250 kyr prior to the K/Pg boundary associated with the incursion of a younger (more enriched δ13C) deep water mass, although total abundances of planktic foraminifera in the sediment remained a tiny fraction of their earlier Maastrichtian values. A second, brief dissolution event occurred ~ 200 kyr before the boundary evidenced by renewed increase in planktic fragmentation, but without a decrease in P/B ratio. Our data show that changing deep water masses, coupled with reduced productivity and associated decrease in pelagic carbonate flux was responsible for the first ~ 1.6-Myr dissolution interval, while Deccan Traps volcanism (?) may have caused surface ocean acidification ~ 200 kyr prior to the K/Pg mass extinction event.

Continue reading ‘Extinction, dissolution, and possible ocean acidification prior to the Cretaceous/Paleogene (K/Pg) boundary in the tropical Pacific’

Faunal succession and geochemical analysis of carbonate facies changes along the late Permian mass extinction boundary in the Nanpanjiang Basin, South China: a potential argument for ocean acidification and its implications

The late Permian mass extinction is considered the largest extinction event in Earth’s history with over 90% of marine and 70% of terrestrial species becoming extinct as a result (Lehrmann et al., 2015). The Nanpanjiang Basin in southern China contains multiple drowned carbonate platforms that are a record of the Permian-Triassic boundary. Data of two subsections from the Tianwan section of the Tian’e platform in the Nanpanjiang Basin consist of Permian carbonates, the altered truncation surface of the Permian-Triassic boundary as well as Triassic microbialites. Analysis of 1) faunal succession, 2) faunal dominance, 3) stable isotopes and 4) diagenetic structures contributes to the understanding of the environmental conditions during the late Permian to early Triassic. Data collected shows a trend from skeletal packstone to microbial boundstone from the Permian to Triassic respectively. Stable isotope analysis of δ13C and δ18O data up section both show large excursions at the extinction boundary.

Continue reading ‘Faunal succession and geochemical analysis of carbonate facies changes along the late Permian mass extinction boundary in the Nanpanjiang Basin, South China: a potential argument for ocean acidification and its implications’

Marine paleoclimatic proxies: A shift from qualitative to quantitative estimation of seawater parameters

Understanding past climate during contrasting boundary conditions can help in assessing imminent climate changes. Marine sediments offer a vast archive of past climate. Various indirect methods called proxies are used to infer principal climate parameters like temperature, salinity, productivity, monsoon intensity, ocean circulation, seawater pH, and others, from the marine sediments. The relationship between a climate parameter and marine paleoclimate proxy may vary from region to region. Additionally, the marine proxies are often affected by more than one climate parameter, thus making it difficult to assess the change in any particular parameter from a single proxy. Diagenetic alteration can also significantly affect the parameter-proxy relationship. A growing emphasis now is on quantifying changes in key climatic parameters in the past. Proxies for quantitative estimation of seawater temperature, runoff, sea-level and pH are now fairly well established. Similar robust quantitative proxies for dissolved oxygen concentration and productivity are, however, still being developed. Additionally, the uncertainty associated with quantitative estimation of past climate has to be reduced. Therefore, continuous efforts are being made to develop novel paleoclimate proxies and to evaluate existing proxies in different regions of the world oceans.

Continue reading ‘Marine paleoclimatic proxies: A shift from qualitative to quantitative estimation of seawater parameters’

Calibration of the B/Ca proxy in the planktic foraminifer Orbulina universa to Paleocene seawater conditions

The B/Ca ratio of planktic foraminiferal calcite, a proxy for the surface ocean carbonate system, displays large negative excursions during the Paleocene-Eocene Thermal Maximum (PETM, 55.9 Ma), consistent with rapid ocean acidification at that time. However, the B/Ca excursion measured at the PETM exceeds a magnitude that modern pH-calibrations can explain. Numerous other controls on the proxy have been suggested, including foraminiferal growth rate and the total concentration of Dissolved Inorganic Carbon (DIC). Here we present new calibrations for B/Ca vs. the combined effects of pH and DIC in the symbiont-bearing planktic foraminifer Orbulina universa, grown in culture solutions with simulated Paleocene seawater elemental composition (high [Ca], low [Mg], and low [B]T). We also investigate the isolated effects of low seawater total boron concentration ([B]T), high [Ca], reduced symbiont photosynthetic activity, and average shell growth rate on O. universa B/Ca in order to further understand the proxy systematics and to determine other possible influences on the PETM records. We find that average shell growth rate does not appear to determine B/Ca in high calcite saturation experiments. In addition, our “Paleocene” calibration shows higher sensitivity than the modern calibration at low [B(OH)4-]/DIC. Given a large DIC pulse at the PETM, this amplification of the B/Ca response can more fully explain the PETM B/Ca excursion. However, further calibrations with other foraminifer species are needed to determine the range of foraminifer species-specific proxy sensitivities under these conditions for quantitative reconstruction of large carbon cycle perturbations.

Continue reading ‘Calibration of the B/Ca proxy in the planktic foraminifer Orbulina universa to Paleocene seawater conditions’

Deepwater carbonate ion concentrations in the western tropical Pacific since 250 ka: Evidence for oceanic carbon storage and global climate influence

We present new “size-normalized weight” (SNW)-Δ[CO32−] core-top calibrations for three planktonic foraminiferal species and assess their reliability as a paleo-alkalinity proxy. SNWs of Globigerina sacculifer and Neogloboquadrina dutertrei can be used to reconstruct past deep Pacific [CO32−], whereas SNWs of Pulleniatina obliquiloculata are controlled by additional environmental factors. Based on this methodological advance, we reconstruct SNW-based deepwater [CO32−] for core WP7 from the western tropical Pacific since 250 ka. Secular variation in the SNW proxy documents little change in deep Pacific [CO32−] between the Last Glacial Maximum and the Holocene. Further back in time, deepwater [CO32−] shows long-term increases from marine isotope stage (MIS) 5e to MIS 3 and from early MIS 7 to late MIS 6, consistent with the “coral reef hypothesis” that the deep Pacific Ocean carbonate system responded to declining shelf carbonate production during these two intervals. During deglaciations, we have evidence of [CO32−] peaks coincident with Terminations 2 and 3, which suggests that a breakdown of oceanic vertical stratification drove a net transfer of CO2 from the ocean to the atmosphere, causing spikes in carbonate preservation (i.e., the “deglacial ventilation hypothesis”). During MIS 4, a transient decline in SNW-based [CO32−], along with other reported [CO32−] and/or dissolution records, implies that increased deep-ocean carbon storage resulted in a global carbonate dissolution event. These findings provide new insights into the role of the deep Pacific in the global carbon cycle during the late Quaternary.

Continue reading ‘Deepwater carbonate ion concentrations in the western tropical Pacific since 250 ka: Evidence for oceanic carbon storage and global climate influence’

Calcareous nannoplankton response to the latest Cenomanian Oceanic Anoxic Event 2 perturbation

Morphometric analyses were performed on Biscutum constans, Zeugrhabdotus erectus, Discorhabdus rotatorius and Watznaueria barnesiae specimens from five sections spanning the Cenomanian-Turonian boundary interval including Oceanic Anoxic Event (OAE) 2 (~ 94 Ma). The study provides evidence for size fluctuations and dwarfism of B. constans during OAE 2, followed by a partial recovery at the end of the event: this taxon appears to be the most sensitive species, with similar and coeval size trends in all the analyzed sections. Conversely, morphometry shows negligible or unsystematic coccolith variations in Z. erectus, D. rotatorius and W. barnesiae. The comparison of OAE 2 data with those available for the early Aptian OAE 1a and latest Albian OAE 1d, indicates that B. constans repeatedly underwent size reduction and temporary dwarfism possibly implying that the same paleoenvironmental factors controlled calcification of B. constans during subsequent OAEs although the amplitude of B. constans coccolith reduction is significantly larger for OAE 1a than OAE 2. Paleoceanographic reconstructions suggest that ocean chemistry related to the amount of CO2 and toxic metal concentrations played a central role in B. constans coccolith secretion, while temperature and nutrient availability do not seem to have been crucial. Contrary to OAE 1a, Z. erectus, D. rotatorius and W. barnesiae appear to be substantially unrelated to OAE 2 paleoenvironmental stress, possibly because of different degrees of perturbation.

Continue reading ‘Calcareous nannoplankton response to the latest Cenomanian Oceanic Anoxic Event 2 perturbation’

Hydrographic and ecologic implications of foraminiferal stable isotopic response across the U.S. mid-Atlantic continental shelf during the Paleocene-Eocene Thermal Maximum

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, 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. Although both scenarios are plausible, similar δ13C responses recorded in surface, thermocline, and benthic foraminifera challenge mixed layer taxa migration.

Continue reading ‘Hydrographic and ecologic implications of foraminiferal stable isotopic response across the U.S. mid-Atlantic continental shelf during the Paleocene-Eocene Thermal Maximum’


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