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

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.