Summary of 40Ar/39Ar results from sediment core CRP-2A (Table 1)

40Ar/39Ar analyses of tephra and clasts of volcanic rock provide age constraints for upper parts of the CRP-2A core. Single-crystal laser-fusion analyses of anorthoclase phenocrysts from three tephra-bearing layers yielded the most precise age constraints for CRP-2A. The dated tephra layers are: 1) a 2.7-m-thick interval of pumice and ash layers between 111.5 and 114.2 meters below sea floor (mbsf) (weighted mean age = 21.44 ± 0.05 Ma, +2.2); 2) a concentration of pumice near 193.4 mbsf (23.98 ± 0.13 Ma): and 3) a concentration of pumice near 280 mbsf (24,22 ± 0.03 Ma) (all ages are calibrated relative to Fish Canyon Tuff sanidine at 27.84 Ma). The 111 to 114 mbsf tephra is almost entirely non-reworked, and the 193 mbsf and 280 mbsf tephra concentrations are interpreted as being reworked and redeposited soon after eruption. All three of the tephra ages are therefore considered to be equivalent to depositional ages. The variation in precision of these three age determinations is largely a function of phenocryst size and abundance. The accuracy of these ages is equal to the accuracy of the current calibration of the 40Ar/39Ar methode (about ± 1 %). 40Ar/39Ar results from volcanic clasts provide three additional maximum age constraints for the CRP-2A core. Single-crystal laser-fusion of sanidine phenocrysts from a rhyolitic clast from 294 mbsf yielded a precise maximum depositional age of 24.98 ± 0.08 Ma, and plateau ages of groundmass concentrates from basaltic clasts near 36.02 mbsf and 125.92 mbsf yielded maximum depositional ages of 19.18 ± 0.12 Ma, and 22.56 ± 0.14 Ma, respectively. The 40Ar/39Ar data, in association with biostratigraphic, paleomagnetic, and isotopic age constraints for CRP-2A, confirm interpretation for rapid sedimentation rates in the 36 to 280 mbsf interval, particularly in the 193 to 280 mbsf interval where they support interpretations for sedimentation cycles spanning 100 k.y. intervals. In addition to the 19 to 25 Ma ages measured from thephra layers and clasts, provenance-related ages ranging from 150 to 450 Ma were determined from clasts and individual detrital or xenocrystic crystals from CRP-2A.

针对CRP-2A岩心的上部层位，对火山碎屑（tephra）与火山岩岩屑（clasts）开展的40Ar/39Ar测年分析为其提供了年代约束条件。对3个含火山碎屑层中的歪长石斑晶（anorthoclase phenocrysts）开展单晶激光熔融测年分析，为CRP-2A岩心提供了精度最高的年代约束。本次定年的火山碎屑层包括：1）海底以下111.5~114.2米（meters below sea floor, mbsf）处一套厚度2.7米的浮岩与火山灰层段，其加权平均年龄为21.44±0.05百万年（Ma），误差上限为+2.2；2）193.4 mbsf附近的浮岩富集层，年龄为23.98±0.13 Ma；3）280 mbsf附近的浮岩富集层，年龄为24.22±0.03 Ma。所有年龄均以27.84 Ma的鱼谷凝灰岩透长石（Fish Canyon Tuff sanidine）作为校准标准。111.5~114.2 mbsf处的火山碎屑层几乎未发生再沉积作用，而193.4 mbsf与280 mbsf处的浮岩富集层被认为是喷发后不久经再搬运再沉积形成的。因此，这3套火山碎屑层的测年结果均可视为其沉积年龄。这3次测年结果的精度差异主要取决于斑晶的粒径与丰度。上述测年结果的准确度与当前40Ar/39Ar测年方法的校准精度一致（误差约为±1%）。火山岩岩屑的40Ar/39Ar测年结果，为CRP-2A岩心提供了额外3项最大沉积年代约束。对取自294 mbsf处流纹岩岩屑中的透长石斑晶（sanidine phenocrysts）开展单晶激光熔融测年，得到精确的最大沉积年龄为24.98±0.08 Ma；对36.02 mbsf与125.92 mbsf附近玄武岩岩屑（basaltic clasts）的基质浓缩物（groundmass concentrates）开展坪年龄（plateau ages）测年，分别得到19.18±0.12 Ma与22.56±0.14 Ma的最大沉积年龄。结合CRP-2A岩心的生物地层学、古地磁学与同位素年代约束数据，上述40Ar/39Ar测年数据证实了36~280 mbsf层段存在快速沉积的认识，尤其是在193~280 mbsf层段，该数据支持了沉积旋回跨度为100千年（k.y.）的解释。除了从火山碎屑层与岩屑中测得的19~25 Ma年龄外，还从CRP-2A岩心的岩屑与单个碎屑或捕虏晶（detrital or xenocrystic crystals）中得到了150~450 Ma的物源相关年龄（provenance-related ages）。