Carbon sources and sinks are primary components of the climate system, but their response to external forcing remain unconstrained, especially for past greenhouse climates. Lakes are important carbon sinks that play a key role in the global carbon cycle. In this study, we investigate organic carbon burial processes and the possible role played by astronomical forcing in low to middle latitude lakes in China during the Eocene. Sediment noise modeling of lake level fluctuations in the three basins suggests that TOC maxima coincided with lake level maxima. We suggest that elevated lake levels likely led to stagnation of bottom waters, thereby promoting the development and preservation of organic matter. Total organic carbon (TOC) data spanning the Eocene from three borehole cores of separate basins show a common cyclicity of ∼1.2 Myr. According to the three TOC time series studied, maxima in TOC are linked to maxima in long-term 1.2 Myr obliquity modulation cycles, with long-term 2.4 Myr eccentricity cycles either at a maxima or minima. Our analysis elucidates a likely control on the burial of organic carbon by long-term astronomical climate cycles. The superposition of different orbital cycles may have driven the process of differential enrichment of organic matter by inducing perturbations in the carbon cycle through nonlinear climate effects. Overall, our chemostratigraphy results illustrate the sensitivity of the terrestrial carbon cycle to orbital forcing on geological timescales.
The Madeira mantle plume is proposed to contain a complete package of recycled oceanic lithosphere. It has not been possible, however, to establish if both lower oceanic crust and peridotitic lithospheric mantle are present within the Madeira magma source. This study reports potassium (K) and oxygen isotope data of lavas from the Madeira Archipelago, which show that δ41K (-0.50 ± 0.06‰ to -0.31 ± 0.01‰) correlates with Sr-Nd-Pb-Hf isotopic compositions, confirming the incorporation of recycled crustal material. The increase in δ41K from Madeira's early shield stage lavas (-0.50 ± 0.06‰ to -0.40 ± 0.05‰) to the subsequent post-erosional stage lavas (-0.34 ± 0.04‰ to -0.31 ± 0.01‰) is consistent with the mantle source becoming progressively depleted in recycled upper oceanic crust (converted to eclogite or pyroxenite) through melt extraction with decreasing age. The post-erosional lavas, however, require a source with higher δ41K values than in depleted peridotitic lithospheric mantle. Modeling demonstrates that during dehydration of high-temperature altered lower oceanic crust, the initial heavy K isotopic signature is preserved, consistent with the high δ41K values of the post-erosional lavas being derived from hydrothermally-altered lower oceanic crust in the Madeira plume source. Consequently, K isotopes provide direct evidence that ocean island magma sources can contain both upper and lower recycled oceanic crust. This study provides important constraints on the cause of the temporal changes in geochemical composition of ocean island basalts, the origin of mantle heterogeneity and the recycling of K through subduction zones and the deep mantle.
Calcium-aluminum-rich inclusions (CAIs) commonly observed in chondritic meteorites are the oldest dated solids formed in the Solar System. Short-lived isotope chronologies (26Al-26Mg, 182Hf-182W) suggest a ∼2 Ma gap between the formation of CAIs and the accretion of the final chondrite parent bodies. One thin section, 3.27 cm2 in size, of an ordinary chondrite NWA 3358 (H3.1) studied contains 52 refractory inclusions (CAIs and amoeboid olivine aggregates (AOAs)) comprising 0.14 % of its area, which is the highest abundance of refractory inclusions among non-carbonaceous chondrites containing on average ∼0.009 area % of CAIs and AOAs. In combination with a low chondrule/matrix ratio of ∼1.5, this makes NWA 3358 a unique ordinary chondrite. The aqueously-formed fayalites (Fa>99) in NWA 3358 have the inferred initial 53Mn/55Mn ratio of (5.56 ± 0.44) × 10−6 which is the highest measured value for secondary minerals in chondrites and corresponds to the formation time of ∼1.0–1.5 Ma after CAIs. Based on the 53Mn-53Cr chronology of fayalite formation and the thermal modeling, we infer that the first-generation of an H chondrite parent body, ∼6–12 km in diameter, accreted within 1.0 Ma after formation of CAIs, filling the gap of ∼2 Ma between CAIs and the earliest chondrite parent bodies. This early accretion provides a possible mechanism of CAIs/AOAs storage in the inner solar nebula and could explain the high amount of refractory inclusions in NWA 3358. A later destruction of these first-generation bodies may also explain the presence of CAIs and chondrules of different ages within later formed chondrite parent bodies.