Carbon isotopic ratios (δ 13 C) of C 27 ‐C 33 n ‐alkanes and C 37 alkenes were measured in the surface sediments from Australian sector of Southern Ocean (47.6°–65.5°S). The δ 13 C of C 31 n ‐alkane derived from higher plant wax shows little change along the latitude (−28.7±0.6 ‰), whereas δ 13 C of C 37 alkenes derived from haptophyte algae decreases from −24.6±0.7 ‰ at 47.6°S to −32.1±1.2 ‰ at 63.9°S. Relatively heavy δ 13 C values of C 31 n ‐alkane suggest significant contribution of C4 plant waxes or conifer resin whose δ 13 C of n ‐alkanes are significantly heavier than those of C3 plants. The δ 13 C of total organic carbon exhibits a latitudinal pattern similar to that of C 37 alkenes, suggesting that organic carbon in the sediments is primarily of marine origin.
Abstract We present a long-term seasonal tree ring cellulose oxygen isotope (δ 18 O c ) time series created by analyzing four segments (S1, S2, S3, and S4) per year during the period of 1951–2009 from southeastern Tibetan Plateau. This intraseasonal δ 18 O c reveals the onset and mature phase of the summer monsoon precipitation in this region. Analysis indicates that the δ 18 O c of S1 has the strongest correlation with precipitation during the regional monsoon onset (29–33 pentads, May 21–June 10, r = −0.69), and the δ 18 O c values for S2, S3, and S4 correlate strongly with June, July, and August precipitation, respectively. Combined δ 18 O c of S2, S3, and S4 shows the most robust correlation ( r = −0.82) with the mature-phase monsoon precipitation (June-July-August, JJA), passing rigorous statistical tests for calibration and verification in dendroclimatology. These results demonstrate the feasibility in using long-term intraseasonal δ 18 O c to reconstruct the Asian summer monsoon's intraseasonal variations.
Terrestrial plant‐derived n‐alkanes (C 25– C 35 ) were measured in three piston cores (PC1, PC2 and PC4) in the Sea of Okhotsk covering the last 30 kyrs. Down core profiles of the n‐alkane concentrations and mass accumulation rates (MAR) were characterized by deglacial maxima. In particular, cores PC2 and PC4, which were collected from the central and western Sea of Okhotsk, respectively, show a two‐step increase around the Meltwater Pulse events (MWP) 1A (14.5–13.5 kyr BP) and 1B (about 10 kyr BP). This finding was interpreted by the outflow of terrestrial organic matter from the submerged land shelf to the Sea of Okhotsk through the East Sakhalin Current. This study demonstrated that the sea level rise forced by global warming in the deglaciation period may have caused the enhanced transport of terrestrial organic matter in marginal seas.
Abstract The subarctic Pacific is a high‐nutrient low‐chlorophyll (HNLC) region in which phytoplankton growth is broadly limited by iron (Fe) availability. However, even with Fe limitation, the western subarctic Pacific (WSP) has significant phytoplankton growth and greater seasonal variability in lower trophic levels than the eastern subarctic Pacific. Therefore, differences in Fe supply must explain the west‐to‐east decrease in seasonal phytoplankton growth. The Fe flux to the euphotic zone in the WSP occurs at a “moderate” value, in that it is significantly higher than its value on the eastern side, yet it is not sufficient enough to cause widespread macronutrient depletion, that is, HNLC status is maintained. Although we recognize several Fe supply processes in the WSP, the mechanisms that account for this moderate value of Fe supply have not previously been explained. Here we demonstrate the pivotal role of tidal mixing in the Kuril Islands chain (KIC) for determining the moderate value. A basin‐scale meridional Fe section shows that Fe derived from sediments in the Sea of Okhotsk is discharged through the KIC into the intermediate water masses (~ 800 m) of the western North Pacific. The redistribution of this Fe‐rich intermediate water by intensive mixing as it crosses the KIC is the predominant process determining the ratio of micronutrient (Fe) to macronutrients (e.g., nitrate) in subsurface waters. This ratio can quantitatively explain the differences in surface macronutrient consumption between the western and eastern subarctic, as well as the general formation and biogeochemistry of HNLC waters of the subarctic North Pacific.
The Maunder Minimum (A.D. 1645–1715) is a useful period to investigate possible sun–climate linkages as sunspots became exceedingly rare and the characteristics of solar cycles were different from those of today. Here, we report annual variations in the oxygen isotopic composition ( δ 18 O) of tree-ring cellulose in central Japan during the Maunder Minimum. We were able to explore possible sun–climate connections through high-temporal resolution solar activity (radiocarbon contents; Δ 14 C) and climate ( δ 18 O) isotope records derived from annual tree rings. The tree-ring δ 18 O record in Japan shows distinct negative δ 18 O spikes (wetter rainy seasons) coinciding with rapid cooling in Greenland and with decreases in Northern Hemisphere mean temperature at around minima of decadal solar cycles. We have determined that the climate signals in all three records strongly correlate with changes in the polarity of solar dipole magnetic field, suggesting a causal link to galactic cosmic rays (GCRs). These findings are further supported by a comparison between the interannual patterns of tree-ring δ 18 O record and the GCR flux reconstructed by an ice-core 10 Be record. Therefore, the variation of GCR flux associated with the multidecadal cycles of solar magnetic field seem to be causally related to the significant and widespread climate changes at least during the Maunder Minimum.
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