Magnetic records from latest Triassic to earliest Jurassic red beds, Utah and Arizona, and from mid-Pleistocene lake beds, New Mexico

Paleomagnetic data obtained from the Upper Triassic to Lower Jurassic strata of the Moenave Formation, southern Utah and northern Arizona, were used to construct a composite magnetostratigraphy and further refine the position of the base of the Jurassic in the southwest U.S.A. The composite magnetostratigraphy provides a chronostratigraphic framework to tie Triassic-Jurassic sedimentation in the southwest U.S.A to marine strata in the United Kingdom, Turkey, and Italy, and to the Pangean rift history including extrusive igneous rocks, preserved in Morocco, and in the Newark Basin, northeast U.S.A. In addition, paleomagnetic data from the Moenave Formation were used to calculate a pole position for North America for the latest Triassic to earliest Jurassic time. A lesser amount of inclination error, flattening factor of 0.78, is record in Moenave Formation strata compared to observation from coeval Newark Basin strata. The new paleomagnetic pole position for North America, corrected for inclination error and 4° of clockwise Colorado Plateau rotation is located at 62.3° N, 68.0° E (A95 = 7.4°, N = 102). Pole positions from the southwest U.S.A. continue to indicate a westerly pole position for North America at the latest Triassic to earliest Jurassic time. A mid-Pleistocene lake sedimentary record obtained from the Valles Caldera, northern New Mexico was investigated using rock magnetic and paleomagnetic techniques. Lake sediments span three glacial and two interglacial intervals, MIS 14 – 10. Both detrital and diagenetic phases are preserved in sediment throughout the core. Preservation of detrital phases indicates well mixed lake conditions were more common during interglacial intervals. Discrete intervals of diagenetic phases indicate anoxic conditions are more common in sediments deposited during glacial intervals. A series of anoxic intervals are identified in sediment deposited during MIS 12 that are closely related to interstadial events characterized by increased Cyperaceae and Juniper pollen counts and increased mean annual temperatures. Paleomagnetic data are mostly normal polarity consistent with Brunhes normal polarity chron. However, paleomagnetic data combined with relative paleointensity records support the presence of three geomagnetic field phenomena 14±/Calabrian Ridge II at ~536 ka, 11± at ~400 ka, and Levantine at ~360-360 ka

蓮華寺池と西湖 : 石野雲嶺の風景

The potential for increased drought frequency and severity linked to anthropogenic climate change in the semi-arid regions of the southwestern United States (US) is a serious concern1. Multi-year droughts during the instrumental period2 and decadal-length droughts of the past two millennia1, 3 were shorter and climatically different from the future permanent, ‘dust-bowl-like’ megadrought conditions, lasting decades to a century, that are predicted as a consequence of warming4. So far, it has been unclear whether or not such megadroughts occurred in the southwestern US, and, if so, with what regularity and intensity. Here we show that periods of aridity lasting centuries to millennia occurred in the southwestern US during mid-Pleistocene interglacials. Using molecular palaeotemperature proxies5 to reconstruct the mean annual temperature (MAT) in mid-Pleistocene lacustrine sediment from the Valles Caldera, New Mexico, we found that the driest conditions occurred during the warmest phases of interglacials, when the MAT was comparable to or higher than the modern MAT. A collapse of drought-tolerant C4 plant communities during these warm, dry intervals indicates a significant reduction in summer precipitation, possibly in response to a poleward migration of the subtropical dry zone. Three MAT cycles ~2 °C in amplitude occurred within Marine Isotope Stage (MIS) 11 and seem to correspond to the muted precessional cycles within this interglacial. In comparison with MIS 11, MIS 13 experienced higher precessional-cycle amplitudes, larger variations in MAT (4–6 °C) and a longer period of extended warmth, suggesting that local insolation variations were important to interglacial climatic variability in the southwestern US. Comparison of the early MIS 11 climate record with the Holocene record shows many similarities and implies that, in the absence of anthropogenic forcing, the region should be entering a cooler and wetter phase