Helen V. McGregor

Diagenesis and geochemistry of porites corals from Papua New Guinea: Implications for paleoclimate reconstruction

Coral proxy records of sea surface temperature (SST) and hydrological balance have become important tools in the field of tropical paleoclimatology. However, coral aragonite is subject to post- depositional diagenetic alteration in both the marine and vadose environments. To understand the impact of diagenesis on coral climate proxies, two mid-Holocene Porites corals from raised reefs on Muschu Island, Papua New Guinea, were analysed for Sr/Ca, 18 O, and 13 C along transects from 100% aragonite to 100% calcite. Thin-section analysis showed a characteristic vadose zone diagenetic sequence, beginning with leaching of primary aragonite and fine calcite overgrowths, transitional to calcite void filling and neomorphic, fabric selective replacement of the coral skeleton. Average calcite Sr/Ca and 18 O values were lower than those for coral aragonite, decreasing from 0.0088 to 0.0021 and 5.2 to 8.1‰, respectively. The relatively low Sr/Ca of the secondary calcite reflects the Sr/Ca of dissolving phases and the large difference between aragonite and calcite Sr/Ca partition coefficients. The decrease in 18 O of calcite relative to coral aragonite is a function of the 18 O of precipitation. Carbon-isotope ratios in secondary calcite are variable, though generally lower relative to aragonite, ranging from 2.5 to 10.4%. The variability of 13 C in secondary calcite reflects the amount of soil CO 2 contributing 13 C-depleted carbon to the precipitating fluids. Diagenesis has a greater impact on Sr/Ca than on 18 O; the calcite compositions reported here convert to SST anomalies of 115°C and 14°C, respectively. Based on calcite Sr/Ca compositions in this study and in the literature, the sensitivity of coral Sr/Ca-SST to vadose-zone calcite diagenesis is 1.1 to 1.5°C per percent calcite. In contrast, the rate of change in coral 18 O-SST is relatively small (0.2 to 0.2°C per percent calcite). We show that large shifts in 18 O, reported for mid-Holocene and Last Interglacial corals with warmer than present Sr/Ca-SSTs, cannot be caused by calcite diagenesis. Low-level calcite diagenesis can be detected through X-ray diffraction techniques, thin section analysis, and high spatial resolution sampling of the coral skeleton and thus should not impede the production of accurate coral paleoclimate reconstructions. Copyright

Mid-Holocene ENSO: Issues in quantitative model-proxy data comparisons

(1) Evaluation of climate model simulations using observed data contributes to the assessment of confidence in model predictions of future climate change. The mid-Holocene represents an opportunity to evaluate model simulations of El Nino-Southern Oscillation (ENSO) in comparison with coral proxy evidence of reduced ENSO amplitude. Quantitative comparisons between coral records and model output have been limited by (1) the use of different measures of ENSO amplitude, (2) possible sampling of natural variability in short records, and (3) uncertainty about the stationarity of the relationship between central Pacific sea surface temperature (SST) variability and ENSO signals at the coral site. We examine these issues using modern and fossil coral records from the western Pacific and model simulations of preindustrial and mid-Holocene climate. As a measure of ENSO amplitude, the standard deviation is found to be preferable to event frequency or size as event-based measures are highly dependent on the choice of threshold and may be unreliable for a small number of events. Model ENSO amplitude is found to be strongly dependent on the choice of averaging period, with calendar year averages smoothing the seasonal ENSO signal. A relatively robust relationship between SST variability in the NINO3.4 region and the ENSO SST and precipitation anomalies archived in corals is demonstrated for the instrumental period and for a set of model simulations. Remaining uncertainty about changes in ENSO teleconnections under paleoclimate conditions implies the need for additional proxy records from ENSO- sensitive regions before quantitative reconstructions of ENSO amplitude can be used to evaluate model sensitivity.

Paleoclimate Data-Model Comparison and the Role of Climate Forcings over the Past 1500 Years*

The past 1500 years provide a valuable opportunity to study the response of the climate system to external forcings. However, the integration of paleoclimate proxies with climate modeling is critical to improving the understanding of climate dynamics. In this paper, a climate system model and proxy records are therefore used to study the role of natural and anthropogenic forcings in driving the global climate. The inverse and forward approaches to paleoclimate data–model comparison are applied, and sources of uncertainty are identified and discussed. In the first of two case studies, the climate model simulations are compared with multiproxy temperature reconstructions. Robust solar and volcanic signals are detected in Southern Hemisphere temperatures, with a possible volcanic signal detected in the Northern Hemisphere. The anthropogenic signal dominates during the industrial period. It is also found that seasonal and geographical biases may cause multiproxy reconstructions to overestimate the magnitude of the long-term preindustrial cooling trend. Inthesecondcasestudy,themodelsimulationsarecomparedwithacorald 18 OrecordfromthecentralPacific Ocean. It is found that greenhouse gases, solar irradiance, and volcanic eruptions all influence the mean state of the central Pacific, but there is no evidence that natural or anthropogenic forcings have any systematic impact on El Ni~ Oscillation. The proxy climate relationship is found to change over time, challenging the assumption of stationarity that underlies the interpretation of paleoclimate proxies. These case studies demonstrate the value of paleoclimate data–model comparison but also highlight the limitations of current techniques and demonstrate the need to develop alternative approaches.

Mid-Pacific microatolls record sea-level stability over the past 5000 yr

There has been geographical variation in sea level since rapid postglacial melting of polar ice ceased similar to 6 k.y. ago, reflecting isostatic adjustments of Earth and ocean surfaces to past (and ongoing) redistribution of ice and water loads. A new data set of over 100 fossil microatolls from Christmas (Kiritimati) Island provides a Holocene sea-level record of unparalleled continuity. Living reef-flat corals grow up to a low-tide level. Adjacent fossil microatolls, long-lived Porites corals up to several meters in diameter, occur at similar elevations (±0.1 m), and extensive fossil microatolls in the island interior are at consistent elevations within each population. Collectively, they comprise an almost continuous sequence spanning the past 5 k.y., indicating that, locally, sea level has been within 0.25 m of its present position, and precluding global sea-level oscillations of one or more meters inferred from less stable locations, or using other sea-level indicators. This mid-Pacific atoll is tectonically stable and far from former ice sheets. The precisely surveyed and radiometrically dated microatolls indicate that sea level has not experienced significant oscillations, in accordance with geophysical modeling, which implies that the eustatic contribution from past ice melt and the isostatic adjustment of the ocean floor to loading largely cancel each other at this site.

Early onset of industrial-era warming across the oceans and continents

The evolution of industrial-era warming across the continents and oceans provides a context for future climate change and is important for determining climate sensitivity and the processes that control regional warming. Here we use post-ad 1500 palaeoclimate records to show that sustained industrial-era warming of the tropical oceans first developed during the mid-nineteenth century and was nearly synchronous with Northern Hemisphere continental warming. The early onset of sustained, significant warming in palaeoclimate records and model simulations suggests that greenhouse forcing of industrial-era warming commenced as early as the mid-nineteenth century and included an enhanced equatorial ocean response mechanism. The development of Southern Hemisphere warming is delayed in reconstructions, but this apparent delay is not reproduced in climate simulations. Our findings imply that instrumental records are too short to comprehensively assess anthropogenic climate change and that, in some regions, about 180 years of industrial-era warming has already caused surface temperatures to emerge above pre-industrial values, even when taking natural variability into account.

Images of diagenetic textures in Porites corals from Papua New Guinea and Indonesia

Diagenesis is now recognized as a potentially major source of error in paleoclimatic reconstructions from fossil and modern coral geochemical records. Key to avoiding spurious results caused by diagenesis is thorough screening of coral material prior to geochemical analysis. In this data brief we present color images from thin sections of fossil and modern Porites corals and demonstrate the effectiveness of thin sections in detecting low levels of diagenesis. The images presented here cover a range of coral preservation levels from pristine aragonite to 100% calcite. We particularly focus on samples containing around 1% diagenetic material, a level known to create artifacts in key climate parameters such as sea surface temperature, and close to the detection limits of other screening methods such as X-ray diffraction (XRD). A qualitative scheme is also presented to rate the degree of diagenesis in a coral, where XRD results are not available or where secondary aragonite is present. Overall, this collection of images is designed as a starting point, in combination with other techniques, to assist in identifying and screening corals for diagenesis.

Copenhagen, Climate Science and the Emotional Geographies of Climate Change

Abstract The Pacific island nation-state of Tuvalu featured significantly at the Copenhagen Conference of Parties (COP) 15 climate change negotiations, where the vulnerability of Tuvalu to sea level rise and emotional outpourings of the Tuvaluan delegation contributed to the nations prominence. In this paper we discuss the likely impacts for Tuvalu of a 1.5°C versus 2°C global warming target and explore sadness and discomfort surrounding discussion of these targets during COP 15. We highlight tensions between science and emotion, arguing that affective encounters can be significant in climate change decision-making. Weeping by a member of the Tuvalu delegation evoked discomfort in the conference plenary. This discomfort briefly unsettled the apparently stable boundaries of convention and protocol that seek to separate emotion from science and politics. We argue that possibilities for change arise when emotions enter climate change negotiations, even though (or perhaps because) these are arenas that privilege rational exchange. Our conclusion urges that more attention be paid to how climate science and emotion are intertwined in climate change politics.

Intensification of the meridional temperature gradient in the Great Barrier Reef following the Last Glacial Maximum

Tropical south-western Pacific temperatures are of vital importance to the Great Barrier Reef (GBR), but the role of sea surface temperatures (SSTs) in the growth of the GBR since the Last Glacial Maximum remains largely unknown. Here we present records of Sr/Ca and δ18O for Last Glacial Maximum and deglacial corals that show a considerably steeper meridional SST gradient than the present day in the central GBR. We find a 1–2 °C larger temperature decrease between 17° and 20°S about 20,000 to 13,000 years ago. The result is best explained by the northward expansion of cooler subtropical waters due to a weakening of the South Pacific gyre and East Australian Current. Our findings indicate that the GBR experienced substantial meridional temperature change during the last deglaciation, and serve to explain anomalous deglacial drying of northeastern Australia. Overall, the GBR developed through significant SST change and may be more resilient than previously thought.

Using communities of practice to enhance interdisciplinary teaching: lessons from four Australian institutions

We report on the establishment of communities of practice at four Australian institutions and evaluate their effectiveness and durability as a means of building staff and institutional capacity for interdisciplinary teaching. A community of practice approach is a potentially valuable methodology for overcoming dynamics of fragmentation, isolation and competition within universities. The communities we established were anchored by a shared focus on the topic of climate change and they worked collaboratively to build relationships of trust and reciprocity between teachers in a wide range of disciplines. The aim of each community was to improve the teaching of climate change through enabling members to integrate diverse disciplinary perspectives, to teach collaboratively, to promote innovation through exchange and to demonstrate leadership within their institutions. The key factors that made our communities effective and durable are: (1) designation of two leadership roles, activator and facilitator, (2) provision for institutional autonomy in domesticating the model to fit local circumstances and (3) a pragmatic emphasis on opportunities for teaching innovation and leadership within existing administrative structures, teaching programs and workloads. We conclude that suitably designed and resourced communities of practice are a viable means of improving interdisciplinary teaching of complex problems by facilitating both staff development and institutional learning.

A global multiproxy database for temperature reconstructions of the Common Era

Reproducible climate reconstructions of the Common Era (1 CE to present) are key to placing industrial-era warming into the context of natural climatic variability. Here we present a community-sourced database of temperature-sensitive proxy records from the PAGES2k initiative. The database gathers 692 records from 648 locations, including all continental regions and major ocean basins. The records are from trees, ice, sediment, corals, speleothems, documentary evidence, and other archives. They range in length from 50 to 2000 years, with a median of 547 years, while temporal resolution ranges from biweekly to centennial. Nearly half of the proxy time series are significantly correlated with HadCRUT4.2 surface temperature over the period 1850–2014. Global temperature composites show a remarkable degree of coherence between high- and low-resolution archives, with broadly similar patterns across archive types, terrestrial versus marine locations, and screening criteria. The database is suited to investigations of global and regional temperature variability over the Common Era, and is shared in the Linked Paleo Data (LiPD) format, including serializations in Matlab, R and Python.