Do Southern Hemisphere tree rings record past volcanic events?

Much of our knowledge about the impacts of volcanic events on climate comes from proxy records. However, little is known about the impact of volcanoes on trees from the Southern Hemisphere. We investigated whether volcanic signals could be identified in ring widths from eight New Zealand dendrochronological species, using superposed epoch analysis. We found that most species are good recorders of volcanic dimming and that the magnitude and persistence of the post-event response can be broadly linked to plant life history traits. Across species, site-based factors, particularly altitude and exposure to prevailing conditions, are more important determinants of the strength of the volcanic response than the species. We then investigated whether proxy selection impacts the magnitude of post-volcanic cooling in tree-ring based temperature reconstructions by developing two new multispecies reconstructions of New Zealand summer (December–February) temperature. Both reconstructions showed temperature anomalies remarkably consistent with studies based on instrumental temperature, and with the ensemble mean response of climate models, demonstrating that New Zealand ring widths are reliable indicators of regional volcanic climate response. However, we also found that volcanic response is complex, with positive, negative, and neutral responses identified – sometimes within the same species group. Species-wide composites thus tend to underestimate the volcanic response. The has important implications for the development of future tree ring and multiproxy temperature reconstructions from the Southern Hemisphere.

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>

Paleo-environmental reconstruction of a Late Quaternary organic-rich section preserved near Ohakune, central North Island, New Zealand

<p>A 6m thick section of organic-rich sediment, exposed at Karioi, near Ohakune, central North Island (672m above sea level), presents an opportunity to form a detailed palynological record of Late Quaternary vegetation and climate change. The organic-rich sequence at Karioi lies beneath a 3.29m thick cover-bed sequence that contains towards its base the c. 25.4 ka cal BP Kawakawa/Oruanui Tephra, a key chronostratigraphic marker for the Last Glacial Maximum (LGM) throughout New Zealand. A previous palynological investigation of the underlying organic sediments suggested they extended back from the LGM (Marine isotope stage 2) to the previous interglacial (MIS 5). Such apparently continuous terrestrial records spanning this age range and located at this altitude are rare. A key feature of the Karioi organic sequence is the occurrence of numerous millimetre- to decimetre- thick tephra, derived from a variety of North Island eruptive sources. The possibility that volcanic processes have influenced vegetation change makes climate inferences at this important site potentially problematic. In this new study of the Karioi section, centimetre-scale palynological and diatom sampling conducted above and below three selected tephra (here named ‘Big Lower Lapilli’, ’Unknown’ tephra, and ‘Little’ tephra) at Karioi, were used to assess the influence of these volcanic events on the vegetation and local hydrology. Loss-on-ignition and magnetic susceptibility were used, alongside pollen and diatom analysis, to infer changes in local hydrology and depositional processes in relation to environmental stability. Together, these analyses helped determine the volcanic impacts on vegetation assemblages gained from the pollen record at the site and allowed these to be disassociated from larger scale climate influences of interest. The results of this study indicate a discernible volcanic impact on vegetation and hydrology following just one of the three volcanic events targeted in the record. High-resolution (0.5cm) pollen analysis above and below the largest of the three tephra layers, the 22cm thick ‘Big Lower Lapilli’ showed a notable change in vegetation assemblage immediately following tephra deposition. The most significant of these changes was the marked increase in herbs. This was an unexpected result thought to be due to the proximity of the site to sub-alpine and alpine herbaceous communities, which in turn were closer to the source of volcanism than other vegetation communities depicted in the pollen record. The changes to the pollen spectra are estimated to have taken 300 years to return to pre-eruption assemblages. Magnetic susceptibility and loss-on-ignition results further add to this research by indicating the comparative stability of the depositional environment around the time of deposition of the ‘Big Lower Lapilli’. Statistical analysis further identified a change in vegetation communities associated with tephra deposition, coinciding with an increase in diatom species abundance, which signified an increase in water volume and depth at the site. This was most clearly seen by the marked increase in Aulacoseira ambigua, which is almost exclusively found in water bodies of at least 2 metres depth. These results have major implications for pollen-based climate reconstructions from sequences with interbedded tephra layers. First, such investigations should include fine resolution analyses around prominent tephra layers to test for possible volcanic disturbance that may be a confounding factor in any paleoclimatic reconstructions applied. In this study, for example, vegetation assemblages may have taken up to 300 years to return to pre-eruption levels, but this recovery phase was well within the c. 1000 year inter-sample period of the original coarse (10cm) resolution record. Without the fine resolution study conducted here, the decline of shrubs and increase in grasses, with no obvious changes to trees following deposition of the ‘Big Lower Lapilli’ could have been inferred as a short-term cooling interval. Beyond this restricted zone of volcanic disturbance, greater confidence in the paleoclimatic interpretation of the Karioi pollen record has been achieved as a result of this finer resolution ‘test’ for volcanic disturbance. Second, the volcanic disturbance indicated following the ‘big lower lapilli’ has shed light on pollen taphonomic sources and pathways at this site and in turn, on spatial patterns of vegetation communities. In this case, the increase in tree pollen relative to non-arboreal pollen is interpreted as originating from more distant forest stands that have been comparatively less affected by the deposition of tephra than locally growing vegetation.</p>

Paleo-environmental reconstruction of a Late Quaternary organic-rich section preserved near Ohakune, central North Island, New Zealand

<p>A 6m thick section of organic-rich sediment, exposed at Karioi, near Ohakune, central North Island (672m above sea level), presents an opportunity to form a detailed palynological record of Late Quaternary vegetation and climate change. The organic-rich sequence at Karioi lies beneath a 3.29m thick cover-bed sequence that contains towards its base the c. 25.4 ka cal BP Kawakawa/Oruanui Tephra, a key chronostratigraphic marker for the Last Glacial Maximum (LGM) throughout New Zealand. A previous palynological investigation of the underlying organic sediments suggested they extended back from the LGM (Marine isotope stage 2) to the previous interglacial (MIS 5). Such apparently continuous terrestrial records spanning this age range and located at this altitude are rare. A key feature of the Karioi organic sequence is the occurrence of numerous millimetre- to decimetre- thick tephra, derived from a variety of North Island eruptive sources. The possibility that volcanic processes have influenced vegetation change makes climate inferences at this important site potentially problematic. In this new study of the Karioi section, centimetre-scale palynological and diatom sampling conducted above and below three selected tephra (here named ‘Big Lower Lapilli’, ’Unknown’ tephra, and ‘Little’ tephra) at Karioi, were used to assess the influence of these volcanic events on the vegetation and local hydrology. Loss-on-ignition and magnetic susceptibility were used, alongside pollen and diatom analysis, to infer changes in local hydrology and depositional processes in relation to environmental stability. Together, these analyses helped determine the volcanic impacts on vegetation assemblages gained from the pollen record at the site and allowed these to be disassociated from larger scale climate influences of interest. The results of this study indicate a discernible volcanic impact on vegetation and hydrology following just one of the three volcanic events targeted in the record. High-resolution (0.5cm) pollen analysis above and below the largest of the three tephra layers, the 22cm thick ‘Big Lower Lapilli’ showed a notable change in vegetation assemblage immediately following tephra deposition. The most significant of these changes was the marked increase in herbs. This was an unexpected result thought to be due to the proximity of the site to sub-alpine and alpine herbaceous communities, which in turn were closer to the source of volcanism than other vegetation communities depicted in the pollen record. The changes to the pollen spectra are estimated to have taken 300 years to return to pre-eruption assemblages. Magnetic susceptibility and loss-on-ignition results further add to this research by indicating the comparative stability of the depositional environment around the time of deposition of the ‘Big Lower Lapilli’. Statistical analysis further identified a change in vegetation communities associated with tephra deposition, coinciding with an increase in diatom species abundance, which signified an increase in water volume and depth at the site. This was most clearly seen by the marked increase in Aulacoseira ambigua, which is almost exclusively found in water bodies of at least 2 metres depth. These results have major implications for pollen-based climate reconstructions from sequences with interbedded tephra layers. First, such investigations should include fine resolution analyses around prominent tephra layers to test for possible volcanic disturbance that may be a confounding factor in any paleoclimatic reconstructions applied. In this study, for example, vegetation assemblages may have taken up to 300 years to return to pre-eruption levels, but this recovery phase was well within the c. 1000 year inter-sample period of the original coarse (10cm) resolution record. Without the fine resolution study conducted here, the decline of shrubs and increase in grasses, with no obvious changes to trees following deposition of the ‘Big Lower Lapilli’ could have been inferred as a short-term cooling interval. Beyond this restricted zone of volcanic disturbance, greater confidence in the paleoclimatic interpretation of the Karioi pollen record has been achieved as a result of this finer resolution ‘test’ for volcanic disturbance. Second, the volcanic disturbance indicated following the ‘big lower lapilli’ has shed light on pollen taphonomic sources and pathways at this site and in turn, on spatial patterns of vegetation communities. In this case, the increase in tree pollen relative to non-arboreal pollen is interpreted as originating from more distant forest stands that have been comparatively less affected by the deposition of tephra than locally growing vegetation.</p>

Supplemental Material: Solid as a rock: Tectonic control of graben extension and dike propagation

Section S1 (chronology of seismic and volcanic events throughout the rifting episode), Section S2 (methods used for generation and processing of the remote sensing data that were employed to generate the DEMs used in this study), and Section S3 (methods for generation of detailed structural maps from the high-resolution orthophotos).<br>

Supplemental Material: Solid as a rock: Tectonic control of graben extension and dike propagation

Section S1 (chronology of seismic and volcanic events throughout the rifting episode), Section S2 (methods used for generation and processing of the remote sensing data that were employed to generate the DEMs used in this study), and Section S3 (methods for generation of detailed structural maps from the high-resolution orthophotos).<br>

Permanent, seasonal, and episodic seismic sources around Vatnajökull, Iceland, from the analysis of correlograms

In this study, we locate and characterise the main seismic noise sources in the region of the Vatnajökull icecap (Iceland). Vatnajökull is the largest Icelandic icecap, covering several active volcanoes. The seismic context is very complex, with glacial and volcanic events occurring simultaneously and the classification between the two can become cumbersome. Using seismic interferometry on continuous seismic data (2011–2019), we calculate the propagation velocities and locate the main seismic sources by using hyperbolic geometry and a grid-search method. We identify and characterise permanent oceanic sources, seasonal glacial-related sources, and episodic volcanic sources. These results give a better understanding of the background seismic noise sources in this region and could allow the identification of seismic sources associated with potentially threatening events in real-time.

Fingerprinting the Cretaceous-Paleogene boundary impact with Zn isotopes

Numerous geochemical anomalies exist at the K-Pg boundary that indicate the addition of extraterrestrial materials; however, none fingerprint volatilization, a key process that occurs during large bolide impacts. Stable Zn isotopes are an exceptional indicator of volatility-related processes, where partial vaporization of Zn leaves the residuum enriched in its heavy isotopes. Here, we present Zn isotope data for sedimentary rock layers of the K-Pg boundary, which display heavier Zn isotope compositions and lower Zn concentrations relative to surrounding sedimentary rocks, the carbonate platform at the impact site, and most carbonaceous chondrites. Neither volcanic events nor secondary alteration during weathering and diagenesis can explain the Zn concentration and isotope signatures present. The systematically higher Zn isotope values within the boundary layer sediments provide an isotopic fingerprint of partially evaporated material within the K-Pg boundary layer, thus earmarking Zn volatilization during impact and subsequent ejecta transport associated with an impact at the K-Pg.