Extreme hydrologic events such as storms and floods have the potential to severely impact modern human society. However, the frequency of storms and their underlying mechanisms are limited by a paucity of suitable proxies, especially in inland areas. Here we present a record of speleothem magnetic minerals to reconstruct paleoprecipitation, including storms, in the eastern Asian monsoon area over the last 8.6 ky. The geophysical parameter IRMsoft-fluxrepresents the flux of soil-derived magnetic minerals preserved in stalagmite HS4, which we correlate with rainfall amount and intensity. IRMsoft-fluxexhibits relatively higher values before 6.7 ky and after 3.4 ky and lower values in the intervening period, consistent with regional hydrological changes observed in independent records. Abrupt enhancements in the flux of pedogenic magnetite in the stalagmite agree well with the timing of known regional paleofloods and with equatorial El Niño−Southern Oscillation (ENSO) patterns, documenting the occurrence of ENSO-related storms in the Holocene. Spectral power analyses reveal that the storms occur on a significant 500-y cycle, coincident with periodic solar activity and ENSO variance, showing that reinforced (subdued) storms in central China correspond to reduced (increased) solar activity and amplified (damped) ENSO. Thus, the magnetic minerals in speleothem HS4 preserve a record of the cyclic storms controlled by the coupled atmosphere−oceanic circulation driven by solar activity.
SamplingBCC-010 was halved along its growth axis following collection. For this study, a 5 mm thick planar section was cut from the face of one of the stalagmite halves. Two 37 mm-wide strips were then cut from the section such that their long axis was perpendicular to the growth horizons, and these were subsequently subdivided into ~5 mm samples (0.9 cm 3 ).
Age ModelWe combine a previously published age model for BCC-010 with a further two dates obtained for this study (1). The chronology for the interval studied in this paper was constructed using 24 230 Th dates determined by U-Th measurements conducted at the Minnesota Isotope Laboratory on the opposing stalagmite half to that used for magnetic analysis (Table DR1).We achieved chemical separation of the uranium and thorium fractions using iron co-precipitation and anion-exchange column chemistry (2). Measurements were carried out on a multi-collector inductively coupled plasma spectrometer (Thermo-Finnigan NEPTUNE). All the samples were spiked with a 233 U-236 U double spike to monitor instrumental mass fractionation. We correct for abundance sensitivity by characterizing the tail correction at masses 234.5 and 237 on the uranium isotope standard, CRM-112A.For those 230 Th dates obtained previously, all uranium isotopes were measured on Faraday cups with the lowest uranium isotope beam intensity ( 234 U) at ~30 mV (1). For the two oldest 230 Th dates, measured for this study, all isotope measurements were performed in peak-jumping mode using a Secondary Electron Multiplier (SEM), with the highest uranium isotope beam intensity ( 235 U) at ~270,000 cps
GSA DATA REPOSITORY 2015207Bourne et al.
Th xs 0 , from user-supplied datasets of uranium and thorium isotope activities from sedimentary samples following the theoretical approach described. 'XSage' can determine variations in sedimentation rate between stratigraphic horizons of known age and thus produce high-resolution age models. Using a Monte Carlo approach, the program calculates uncertainties for these age models and on the durations of intervals between tie-points. An example of the application of the XSage program using a previously published record is provided.Components: 6000 words, 4 figures.
Geomagnetic excursions are recognised as intrinsic features of the Earth's magnetic field. High-resolution records of field behaviour, captured in marine sedimentary cores, present an opportunity to determine the temporal and geometric character of the field during geomagnetic excursions and provide constraints on the mechanisms producing field variability. We present here the highest resolution record yet published of the Blake geomagnetic excursion (∼125 ka) measured in three cores from Ocean Drilling Program (ODP) Site 1062 on the Blake-Bahama Outer Ridge. The Blake excursion has a controversial structure and timing but these cores have a sufficiently high sedimentation rate (∼10 cm ka −1) to allow detailed reconstruction of the field behaviour at this site during the excursion. Palaeomagnetic measurements of the cores reveal rapid transitions (< 500 years) between the
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