Atmospheric carbon dioxide concentrations and climate are regulated on geological timescales by the balance between carbon input from volcanic and metamorphic outgassing and its removal by weathering feedbacks; these feedbacks involve the erosion of silicate rocks and organic-carbon-bearing rocks. The integrated effect of these processes is reflected in the calcium carbonate compensation depth, which is the oceanic depth at which calcium carbonate is dissolved. Here we present a carbonate accumulation record that covers the past 53 million years from a depth transect in the equatorial Pacific Ocean. The carbonate compensation depth tracks long-term ocean cooling, deepening from 3.0-3.5 kilometres during the early Cenozoic (approximately 55 million years ago) to 4.6 kilometres at present, consistent with an overall Cenozoic increase in weathering. We find large superimposed fluctuations in carbonate compensation depth during the middle and late Eocene. Using Earth system models, we identify changes in weathering and the mode of organic-carbon delivery as two key processes to explain these large-scale Eocene fluctuations of the carbonate compensation depth.

S U M M A R YIt is known that the Hawaiian 1960 lava sometimes yields higher palaeointensities than the IGRF (36.2 µT). In order to clarify the causes, we have performed a comprehensive investigation of palaeointensity measurements on 19 cores from the lava. According to various rock magnetic analyses, they are classified into three groups with degrees of the deuteric oxidation. In Coe's version of the Thellier method, a significantly larger mean palaeointensity of 49.0 ± 9.6 µT (N = 17) was observed similar to previous studies. These palaeointensities showed a dependence on the oxidation indices, which can be explained by thermochemical remanent magnetization (TCRM) acquisition during the natural cooling stage of the lava. Although it is generally difficult to distinguish the TCRM from thermoremanent magnetization (TRM), a lowtemperature demagnetization (LTD) treatment on NRM and TRM in the Thellier experiment seems to allow us to detect the influence of the TCRM on NRM. Another possible tool for TCRM detection is the double-heating technique (DHT) of the Shaw method combined with LTD (LTD-DHT Shaw method). It gave a better average of 39.4 ± 7.9 µT (N = 9), though the success rate was much lower. Most of the inappropriate results were screened by a combination of the double-heating test, the ARM correction and the LTD treatment. If two outliers are further excluded, the average is improved to be 35.7 ± 3.3 µT (N = 7), which agrees well with the expected intensity. The results of both methods emphasize the importance of taking and measuring samples from the parts of various oxidation degrees in a lava flow.

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[1] Sediment dominated convergent margins typically record substantial horizontal shortening often associated with great earthquakes. The convergent margin south of Japan is arguably one of the most extensively investigated margins and previous studies have documented extensive evidence for accretion and horizontal shortening. Here, we show results from anelastic strains recovered from three partially lithified sediment samples ($40% porosities) across the southwest Japan accretionary prism and propose that the margin is dominated by horizontal extension rather than compression. The anelastic strain results are also consistent with stress directions interpreted from two independent techniques -bore hole breakout orientations and core-scale fault data. We interpret this unexpected result to reflect geologically recent underthrusting of a thick sediment package and concomitant weakening of the decollément. Citation: Byrne, T. B.,

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