Taxonomy of Middle Miocene foraminifera from the northern Namibian continental shelf

Middle Miocene foraminifera from the northern Namibian outer continental shelf are indicators of a period prior to the initiation of the Benguela Upwelling System (BUS). This study provides an update to the occurrence and taxonomy of Miocene foraminifera from the continental margin of Namibia. The taxonomy of 51 benthic and 12 planktic foraminiferal species from the northern Namibian shelf are discussed, their stratigraphic significance given, and their ecological preferences and regional distribution summarised within this study. The identification of extinct planktic foraminifera provided key stratigraphic control for the middle Miocene strata of this region. The taxa identified in this study provide a distinct and different assemblage to the overlying younger strata. Many of the species recorded in this study have not been identified in the region and are reported for the first time from the middle Miocene on the southwestern continental shelf of Africa, off Namibia. A total of 47 species are identified and discussed for the first time from this region. Nineteen species recorded in this study are extinct and eleven taxa reported here have previously only been reported on the genus level on the southwestern shelf of South Africa. Seven benthic species (Amphicoryna scalaris, Marginulina obesa, Glandulina laevigata, Globocassidulina subglobosa, Uvigerina peregrina, Sphaeroidina bulloides and Melonis affinis) and two planktic species (Globigerina bulloides and Orbulina universa) did not disappear from the regional stratigraphy and continued to occur in Plio-Pleistocene to Recent sediments along the southwestern continental shelf of Africa.

Biological Response of Planktic Foraminifera to Decline in Seawater pH

Understanding the way in which a decline in ocean pH can affect calcareous organisms could enhance our ability to predict the impacts of the potential decline in seawater pH on marine ecosystems, and could help to reconstruct the paleoceanographic events over a geological time scale. Planktic foraminifera are among the most important biological proxies for these studies; however, the existing research on planktic foraminifera is almost exclusively based on their geochemical indices, without the inclusion of information on their biological development. Through a series of on-board experiments in the western tropical Pacific (134°33′54″ E, 12°32′47″ N), the present study showed that the symbiont-bearing calcifier Trilobatus sacculifer—a planktic foraminifer—responded rapidly to a decline in seawater pH, including losing symbionts, bleaching, etc. Several indices were established to quantify the relationships between these biological parameters and seawater pH, which could be used to reconstruct the paleoceanographic seawater pH. We further postulated that the loss of symbionts in planktic foraminifera acts as an adaptive response to the stress of low pH. Our results indicate that an ongoing decline in seawater pH may hinder the growth and calcification of planktic foraminifera by altering their biological processes. A reduction in carbonate deposition and predation could have profound effects on the carbon cycle and energy flow in the marine food web.

Contrasting vertical distributions of recent planktic foraminifera off Indonesia during the southeast monsoon: implications for paleoceanographic reconstructions

Planktic foraminifera are widely used in palaeoceanographic and paleoclimatic studies. The accuracy of such reconstructions depends on our understanding of the organisms’ ecology. Here we report on field observations of planktic foraminiferal abundances (>150 µm) from 5 depth intervals between 0–500 m water depth at 37 sites in the eastern tropical Indian Ocean. The total planktic foraminiferal assemblage comprises 29 morphospecies; with 11 morphospecies accounting for ~90 % of the total assemblage. Both species composition and dominance in the net samples are broadly consistent with the published data from the corresponding surface sediments. The abundance and vertical distribution of planktic foraminifera are low offshore west Sumatra, and increase towards offshore south Java and the Lesser Sunda Islands (LSI). Average living depth of Trilobatus trilobus, Globigerinoides ruber, and Globigerina bulloides increases eastward, while that of Neogloboquadrina dutertrei, Pulleniatina obliquiloculata, and Globorotalia menardii remains constant. We interpret the overall zonal and vertical distribution patterns in planktic foraminiferal abundances as a response to the contrasting upper water column conditions during the southeast monsoon, i.e., oligotrophic and stratified offshore Sumatra (non-upwelling) vs. eutrophic and well-mixed offshore Java-LSI (upwelling). Overall, the inferred habitat depths of selected planktic foraminifera species show a good agreement with those from sediment trap samples and from surface sedimentss off Sumatra, but not with those from surface sediments off Java-LSI. The discrepancy might stem from the different temporal coverage of these sample types. Our findings highlight the need to consider how foraminiferal assemblages and ecology vary on shorter timescales, i.e., from “snapshots” of the water column captured by plankton net to seasonal and interannual variability as recorded in sediment traps and how these changes are transferred and preserved in deep-sea sediments.

Stable isotopes and predation marks shed new light on ammonoid habitat depth preferences

Ammonoids are extinct cephalopods with external shells which predominated in many late Paleozoic and Mesozoic marine ecosystems. Stable isotope data from ammonoid shells constitute primary tools for understanding their palaeohabitats. However, in most sedimentary successions globally the aragonitic shells of ammonoids are dissolved during fossilisation process and therefore not available for geochemical studies. We overcome this taphonomic bias by analysing the better preservable calcitic elements of the ammonoid jaws (aptychi). We study moulds and aptychi of two successive members, temporal subspecies in our interpretation, of a scaphitid evolutionary lineage from a Late Cretaceous chalk succession in Poland. In order to reconstruct their habitat depth preferences, we apply the powerful combination of stable isotope data from aptychi and co-occurring benthic and planktic foraminifera with an analysis of predation marks preserved on scaphitid specimens. On this basis we conclude that the populations of the older subspecies led a nektic, and those of the younger subspecies, a nektobenthic lifestyle. The shift in habitat depth preferences took place probably as a response of local populations to the shallowing of the sea. Previous studies largely assumed stable depth preferences for ammonoid species, genera and even higher clades. Our study casts doubts over such generalizations by pointing out that ammonoids could have been more flexible in their depth-related behaviour than anticipated.

New Biochronological Scales of Planktic Foraminifera for the Early Danian Based on High-Resolution Biostratigraphy

After the Cretaceous/Paleogene boundary (KPB) catastrophic mass extinction event, an explosive evolutionary radiation of planktic foraminifera took place in consequence of the prompt occupation of empty niches. The rapid evolution of new species makes it possible to establish high-resolution biozonations in the lower Danian. We propose two biostratigraphic scales for low-to-middle latitudes spanning the first two million years of the Danian. The first is based on qualitative data and includes four biozones: the Guembelitria cretacea Zone (Dan1), the Parvularugoglobigerina longiapertura Zone (Dan2), the Parvularugoglobigerina eugubina Zone (Dan3), and the Parasubbotina pseudobulloides Zone (Dan4). The latter two are divided into several sub-biozones: the Parvularugoglobigerina sabina Subzone (Dan3a) and the Eoglobigerina simplicissima Subzone (Dan3b) for the Pv. eugubina Zone, and the Praemurica taurica Subzone (Dan4a), the Subbotina triloculinoides Subzone (Dan4b), and the Globanomalina compressa Subzone (Dan4c) for the P. pseudobulloides Zone. The second scale is based on quantitative data and includes three acme-zones (abundance zones): the Guembelitria Acme-zone (DanAZ1), the Parvularugoglobigerina-Palaeoglobigerina Acme-zone (DanAZ2), and the Woodringina-Chiloguembelina Acme-zone (DanAZ3). Both biozonations are based on high-resolution samplings of the most continuous sections of the lower Danian worldwide and have been calibrated with recent magnetochronological and astrochronological dating.

Impact of large volcanic events on the marine environment recorded in marine cores

<p>Explosive silicic volcanic eruptions blanket widespread terrestrial and marine areas in ash, and have a profound effect on climate and local ecosystems. Short-term climate effects are caused by the dispersal of ash, but the injection of gas into the stratosphere, with sulphur being particularly important, drives a cooling of the climate that can last several years. These prolonged perturbations have been observed and recorded in recent decades, but despite the importance of the ocean in regulating global atmospheric climate, little is known about how and to what extent the climate signal produced by volcanic eruptions alters the oceanic environment. As the composition of foraminifera tests is highly sensitive to changes in the surrounding environment, a significant sea surface temperature decrease following a large silicic volcanic eruption may be recorded in the tests of live planktic foraminifera, now preserved in marine sediments. This study examines marine cores (and foraminifera within) that contain tephra units from three major volcanic events to determine if changes can be resolved in ocean temperature and/or foraminifera test morphology following large silicic eruptions. The Holocene Taupo, Waimihia and Mamaku tephra units have been identified in a series of marine sediment cores collected from areas with high sedimentation rates off the east coast of North Island, New Zealand. The sources of these eruptions were from two calderas within the Taupo Volcanic Zone, one of the most active and important rhyolitic regions in the world. Sampling of sediment and foraminifera from these cores has been undertaken at 0.5 cm intervals above and below each tephra. This equates to varying sampling resolutions between cores of 5-30 years, with sufficient sampling taken to establish a stratigraphic record of >100 years either side of each tephra unit. A detailed stratigraphy was undertaken on the sediment surrounding all tephra units, including grain size and CaCO₃ analyses, to identify primary and secondary tephra deposits. One core, Tan0810-12 that contained solely Taupo tephra, was selected for foraminiferal analyses to determine changes in ocean temperature and foraminifera test morphology following this eruption. This core was selected based on the results of the stratigraphic analyses that identified the tephra as a primary deposit with minimal bioturbation above the ash layer and a very high sedimentation rate that enabled sub-decadal scale sampling. Scanning electron microscope imaging was employed to identify the presence of surface contaminants on and within the foraminifera tests and allowed observations of test morphology and size. The morphologies of planktic foraminifera species Globigerinoides ruber and Globigerina bulloides showed no obvious change following the Taupo eruption. The Globigerinoides ruber test sizes distinctly decreased for a period after the eruption, while Globigerina bulloides tests slightly increased in size, correlating well with a decrease in sea surface temperature after the eruption as these species prefer warmer and colder temperatures, respectively. This suggests there is potential for test size to be employed as a proxy for temperature change in conjunction with geochemical analyses. Mg/Ca temperature analyses were conducted in situ using laser ablation inductively coupled plasma mass spectrometry. Both species indicated a decrease in sea surface temperatures when comparing results from tests collected below the tephra deposit to those above. Further results indicate ocean temperature may not have recovered for more than 65 years after the eruption. Such a rapid change in the oceanic environment not only has drastic implications for marine ecosystems but also atmospheric climate, and therefore, terrestrial ecosystems. To reduce the margin of error and determine a more exact value of temperature change following the eruption a greater population of foraminifera is needed. Nonetheless, this study highlights the potential of this method in determining how the oceans are impacted by volcanism and how further research is needed to determine the effects of volcanic eruptions on past and future climate.</p>

Impact of large volcanic events on the marine environment recorded in marine cores

<p>Explosive silicic volcanic eruptions blanket widespread terrestrial and marine areas in ash, and have a profound effect on climate and local ecosystems. Short-term climate effects are caused by the dispersal of ash, but the injection of gas into the stratosphere, with sulphur being particularly important, drives a cooling of the climate that can last several years. These prolonged perturbations have been observed and recorded in recent decades, but despite the importance of the ocean in regulating global atmospheric climate, little is known about how and to what extent the climate signal produced by volcanic eruptions alters the oceanic environment. As the composition of foraminifera tests is highly sensitive to changes in the surrounding environment, a significant sea surface temperature decrease following a large silicic volcanic eruption may be recorded in the tests of live planktic foraminifera, now preserved in marine sediments. This study examines marine cores (and foraminifera within) that contain tephra units from three major volcanic events to determine if changes can be resolved in ocean temperature and/or foraminifera test morphology following large silicic eruptions. The Holocene Taupo, Waimihia and Mamaku tephra units have been identified in a series of marine sediment cores collected from areas with high sedimentation rates off the east coast of North Island, New Zealand. The sources of these eruptions were from two calderas within the Taupo Volcanic Zone, one of the most active and important rhyolitic regions in the world. Sampling of sediment and foraminifera from these cores has been undertaken at 0.5 cm intervals above and below each tephra. This equates to varying sampling resolutions between cores of 5-30 years, with sufficient sampling taken to establish a stratigraphic record of >100 years either side of each tephra unit. A detailed stratigraphy was undertaken on the sediment surrounding all tephra units, including grain size and CaCO₃ analyses, to identify primary and secondary tephra deposits. One core, Tan0810-12 that contained solely Taupo tephra, was selected for foraminiferal analyses to determine changes in ocean temperature and foraminifera test morphology following this eruption. This core was selected based on the results of the stratigraphic analyses that identified the tephra as a primary deposit with minimal bioturbation above the ash layer and a very high sedimentation rate that enabled sub-decadal scale sampling. Scanning electron microscope imaging was employed to identify the presence of surface contaminants on and within the foraminifera tests and allowed observations of test morphology and size. The morphologies of planktic foraminifera species Globigerinoides ruber and Globigerina bulloides showed no obvious change following the Taupo eruption. The Globigerinoides ruber test sizes distinctly decreased for a period after the eruption, while Globigerina bulloides tests slightly increased in size, correlating well with a decrease in sea surface temperature after the eruption as these species prefer warmer and colder temperatures, respectively. This suggests there is potential for test size to be employed as a proxy for temperature change in conjunction with geochemical analyses. Mg/Ca temperature analyses were conducted in situ using laser ablation inductively coupled plasma mass spectrometry. Both species indicated a decrease in sea surface temperatures when comparing results from tests collected below the tephra deposit to those above. Further results indicate ocean temperature may not have recovered for more than 65 years after the eruption. Such a rapid change in the oceanic environment not only has drastic implications for marine ecosystems but also atmospheric climate, and therefore, terrestrial ecosystems. To reduce the margin of error and determine a more exact value of temperature change following the eruption a greater population of foraminifera is needed. Nonetheless, this study highlights the potential of this method in determining how the oceans are impacted by volcanism and how further research is needed to determine the effects of volcanic eruptions on past and future climate.</p>

Environmental Controls on Coccolithophore Blooms in the Southwest Pacific Ocean during Marine Isotope Stages 5e (125 ka) and 7a (210 ka)

<p>Coccolithophores play a key role in the ocean carbon cycle, regulating the uptake and release of CO2. Satellite observations over the past few decades show ocean change in a warming world is accompanied by changes in the latitudinal distribution of coccolithophore blooms. Despite their importance in the carbon cycle, knowledge of the causes of coccolithophore blooms, and how they may respond to future climate change is limited. In this study evidence from marine sedimentary cores is used to derive longer, more complete records of past coccolithophore productivity, and the factors that potentially caused enhanced coccolithophore productivity in previous interglacials. Carbonate-rich marine cores; subtropical P71 from north of New Zealand (33°51.3‟S, 174°41.6‟E) and subantarctic Ocean Drilling Project (ODP) 1120 from the Campbell Plateau (50°3.803‟S, 173°22.300‟E) show abrupt changes between foraminiferal-rich sediments during glacials to coccolith-rich sediments during interglacials. Both cores encompass the last two complete interglacial cycles, Marine Isotope Stage (MIS) 5 (71-130ka) and MIS 7 (191- 243ka). While MIS 5 has been well-studied in the Southwest Pacific Ocean, research on MIS 7 is limited. From the literature, and data from this study, new insights are presented into the climatic and oceanographic conditions during MIS 7. Sea surface temperatures in the subtropical Tasman Inflow were comparable to present during MIS 7a (191-222ka), but were cooler in MIS 7c (235-243ka), implying a change in flow regime potentially related to the dynamics of the South Pacific Gyre. During MIS 7a and 7c the temperature gradient across the Subtropical Front (STF), which separates subtropical and subantarctic waters, was greater than present on the Chatham Rise, at >2°C per 1° latitude. In the Tasman Sea, the STF moved northwards by ~2° latitude. This thesis employs grain size data and scanning electron microscope images to show that significant coccolithophore blooms occurred during MIS 7a at subtropical core P71, but not during interglacial peak MIS 5e (117-130ka), whilst the reverse is true at subantarctic core ODP 1120. A range of paleo-environmental proxies are used to determine the potential conditions that caused these coccolithophore blooms. This includes mass accumulation rates of CaCO3 and % of <20μm grain size that texturally identifies coccoliths, to determine relative rates of coccolithophore productivity. Oxygen isotopes (δ18O) of multiple planktic and benthic foraminifera provide age models, with the former also helping to identify upper water column stratification. Mg/Ca ratios in planktic foraminifera, Globigerinoides ruber, and Random Forest modelling of planktic foraminifera assemblages have been used to derive paleo-temperature estimates. These methods, coupled with trace element data from G. ruber as a productivity proxy, foraminifera assemblages, data on solar insolation and scanning electron microscope images, collectively determine the oceanic conditions at the time of coccolithophore blooms at each core site. The results suggest that no one factor was responsible for blooming, rather it was the combination, and interactions between different environmental processes, that were important. At P71, key factors for bloom formation in MIS 7a were high insolation, thermal stratification of the uppermost ocean, and well-mixed source waters from the Tasman Inflow. At ODP 1120, blooms in MIS 5e resulted from decreased windiness, warmer sea surface temperatures and reduced oceanic circulation over the Campbell Plateau, resulting in marked thermal stratification. It is likely that coccolithophore blooms further enhanced stratification at each core site, and restricted productivity further down the water column. At P71, modern oceanic trends suggest that conditions that caused blooms during MIS 7a will not be met in the near future, and blooming is unlikely to increase at this core site. At ODP 1120, modern trends are less clear, but future conditions are projected to be comparable to MIS 5e, suggesting that coccolithophore blooming may increase in the future in subantarctic waters.</p>

Environmental Controls on Coccolithophore Blooms in the Southwest Pacific Ocean during Marine Isotope Stages 5e (125 ka) and 7a (210 ka)

<p>Coccolithophores play a key role in the ocean carbon cycle, regulating the uptake and release of CO2. Satellite observations over the past few decades show ocean change in a warming world is accompanied by changes in the latitudinal distribution of coccolithophore blooms. Despite their importance in the carbon cycle, knowledge of the causes of coccolithophore blooms, and how they may respond to future climate change is limited. In this study evidence from marine sedimentary cores is used to derive longer, more complete records of past coccolithophore productivity, and the factors that potentially caused enhanced coccolithophore productivity in previous interglacials. Carbonate-rich marine cores; subtropical P71 from north of New Zealand (33°51.3‟S, 174°41.6‟E) and subantarctic Ocean Drilling Project (ODP) 1120 from the Campbell Plateau (50°3.803‟S, 173°22.300‟E) show abrupt changes between foraminiferal-rich sediments during glacials to coccolith-rich sediments during interglacials. Both cores encompass the last two complete interglacial cycles, Marine Isotope Stage (MIS) 5 (71-130ka) and MIS 7 (191- 243ka). While MIS 5 has been well-studied in the Southwest Pacific Ocean, research on MIS 7 is limited. From the literature, and data from this study, new insights are presented into the climatic and oceanographic conditions during MIS 7. Sea surface temperatures in the subtropical Tasman Inflow were comparable to present during MIS 7a (191-222ka), but were cooler in MIS 7c (235-243ka), implying a change in flow regime potentially related to the dynamics of the South Pacific Gyre. During MIS 7a and 7c the temperature gradient across the Subtropical Front (STF), which separates subtropical and subantarctic waters, was greater than present on the Chatham Rise, at >2°C per 1° latitude. In the Tasman Sea, the STF moved northwards by ~2° latitude. This thesis employs grain size data and scanning electron microscope images to show that significant coccolithophore blooms occurred during MIS 7a at subtropical core P71, but not during interglacial peak MIS 5e (117-130ka), whilst the reverse is true at subantarctic core ODP 1120. A range of paleo-environmental proxies are used to determine the potential conditions that caused these coccolithophore blooms. This includes mass accumulation rates of CaCO3 and % of <20μm grain size that texturally identifies coccoliths, to determine relative rates of coccolithophore productivity. Oxygen isotopes (δ18O) of multiple planktic and benthic foraminifera provide age models, with the former also helping to identify upper water column stratification. Mg/Ca ratios in planktic foraminifera, Globigerinoides ruber, and Random Forest modelling of planktic foraminifera assemblages have been used to derive paleo-temperature estimates. These methods, coupled with trace element data from G. ruber as a productivity proxy, foraminifera assemblages, data on solar insolation and scanning electron microscope images, collectively determine the oceanic conditions at the time of coccolithophore blooms at each core site. The results suggest that no one factor was responsible for blooming, rather it was the combination, and interactions between different environmental processes, that were important. At P71, key factors for bloom formation in MIS 7a were high insolation, thermal stratification of the uppermost ocean, and well-mixed source waters from the Tasman Inflow. At ODP 1120, blooms in MIS 5e resulted from decreased windiness, warmer sea surface temperatures and reduced oceanic circulation over the Campbell Plateau, resulting in marked thermal stratification. It is likely that coccolithophore blooms further enhanced stratification at each core site, and restricted productivity further down the water column. At P71, modern oceanic trends suggest that conditions that caused blooms during MIS 7a will not be met in the near future, and blooming is unlikely to increase at this core site. At ODP 1120, modern trends are less clear, but future conditions are projected to be comparable to MIS 5e, suggesting that coccolithophore blooming may increase in the future in subantarctic waters.</p>