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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[1] The process of data selection in paleointensity studies is an essential step to ensure data fidelity. There is, however, no consensus as to the best approach to consistently select data with most studies using arbitrarily defined thresholds for selection. We present a new numerical model that simulates the variability of paleointensity data from hypothetical ideal samples acquiring a thermoremanent magnetization (TRM) by incorporating experimental noise, which has been constrained using over 75,000 data measurements. Using Monte Carlo analyses, we investigate the behavior of simulated data and characterize the distributions of parameters typically used to select paleointensity data. We use the 95th percentiles of the distributions to define thresholds for the maximum likely parameter values that can result from experimental noise. These represent values below which we cannot distinguish non-ideal behavior from noise. We find that a number of parameters are highly sensitive to noise and laboratory field strength (e.g., partial TRM, pTRM, check CDRAT and pTRM tail check dt*); this sensitivity may diminish their ability to identify non-ideal behavior. The fractional ( f ) dependence of some parameters and the proportion of inaccurate results provide justification for f ≥ 0.35 when selecting data from both Thellier-Thellier and Coe protocol experiments. The manifestation of noise in the original Thellier method, however, is different to that of methods that use zero-field heating steps. This suggests that the data selection procedure for the Thellier method should be different, but it also suggests that, contrary to previous analyses, the accuracy and scatter of results from this method are more sensitive to noise than methods that use zero-field heating steps. The general approach taken here is shown to be a powerful means of understanding the behavior of selection parameters and has the potential to be extended to models incorporating non-ideal behavior resulting from alteration and multidomain grains.

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[1] Pelagic marine carbonates provide important records of past environmental change. We carried out detailed low-temperature magnetic measurements on biogenic magnetite-bearing sediments from the Southern Ocean (Ocean Drilling Program (ODP) Holes 738B, 738C, 689D, and 690C) and on samples containing whole magnetotactic bacteria cells. We document a range of low-temperature magnetic properties, including reversible humped low-temperature cycling (LTC) curves. Different degrees of magnetite oxidation are considered to be responsible for the observed variable shapes of LTC curves. A dipole spring mechanism in magnetosome chains is introduced to explain reversible LTC curves. This dipole spring mechanism is proposed to result from the uniaxial anisotropy that originates from the chain arrangement of biogenic magnetite, similar to published results for uniaxial stable single domain (SD) particles. The dipole spring mechanism reversibly restores the remanence during warming in LTC measurements. This supports a previous idea that remanence of magnetosome chains is completely reversible during LTC experiments. We suggest that this magnetic fingerprint is a diagnostic indicator for intact magnetosome chains, although the presence of isolated uniaxial stable SD particles and magnetically interacting particles can complicate this test. Magnetic measurements through the Eocene section of ODP Hole 738B reveal an interval with distinct magnetic properties that we interpret to originate from less oxidized biogenic magnetite and enrichment of a biogenic "hard" component. Co-occurrence of these two magnetic fingerprints during the late Eocene in the Southern Ocean indicates less oxic conditions, probably due to increased oceanic primary productivity and organic carbon burial.