Geology topics

Matthew Kirby

Publications and source records attributed to Matthew Kirby.

2 recordsLinked to original sources

USGS2024

Inbuilt age, residence time, and inherited age from radiocarbon dates of modern fires and late Holocene deposits, Western Transverse Ranges, California

Radiocarbon dates of sedimentary deposits include the elapsed time between formation of the organic material and deposition at the sample site, known as the inherited age. Long inherited ages reduce the accuracy of estimates of the timing of depositional events used to infer paleoclimate change, fire histories, and paleoearthquake timing. An inherited age distribution combines the inbuilt age distribution, which reflects the age composition of the vegetation of the source area, and the residence time distribution, which includes transport and interim storage prior to final deposition. Differentiating residence time and inbuilt age is difficult given typical dispersion of ages in a sedimentary deposit. We address this problem by comparing charcoal dates from two modern fires in southern California, the 2020 Bobcat and the 2013 Grand Fire, with a well-dated late Holocene deposit in the Pallett Creek watershed. The modern fire deposits have negligible transport time (<1 year), and 56 radiocarbon dates indicate a median age of 25 years (300-year 95% range) provides an estimate of inbuilt age for the San Gabriel Mountains. The inherited age calculated from the paleodeposits is older with a median age of ~90 years and has a positive skew (850-year 95% range). A modeled inherited age, calculated by applying the pre-bomb radiocarbon calibration curve to the modern fire age distribution, is shorter than the paleodeposit inherited age by only 21 years, indicating samples with long residence times are not common in the deposit. Comparison of inherited ages calculated from organic-rich and clastic paleodeposits indicate a slight facies dependence that may reflect longer residence time in clastic deposits. The results provide insight into the transport of charcoal through the landscape are useful for refining estimates of past environmental and tectonic events.

California↗

USGS2018

Placing the Common Era in a Holocene context: Millennial to centennial patterns and trends in the hydroclimate of North America over the past 2000 years

A synthesis of 93 hydrologic records from across North and Central America, and adjacent tropical and Arctic islands, reveals centennial to millennial trends in the regional hydroclimates of the Common Era (CE; past 2000 years). The hydrological records derive from materials stored in lakes, bogs, caves, and ice from extant glaciers, which have the continuity through time to preserve low-frequency ( > 100 year) climate signals that may extend deeper into the Holocene. The most common pattern, represented in 46 (49 %) of the records, indicates that the centuries before 1000 CE were drier than the centuries since that time. Principal component analysis indicates that millennial-scale trends represent the dominant pattern of variance in the southwestern US, northeastern US, mid-continent, Pacific Northwest, Arctic, and tropics, although not all records within a region show the same direction of change. The Pacific Northwest and the southernmost tier of the tropical sites tended to dry toward present, as many other areas became wetter than before. In 22 records (24 %), the Medieval Climate Anomaly period (800–1300 CE) was drier than the Little Ice Age (1400–1900 CE), but in many cases the difference was part of the longer millennial-scale trend, and, in 25 records (27 %), the Medieval Climate Anomaly period represented a pluvial (wet) phase. Where quantitative records permitted a comparison, we found that centennial-scale fluctuations over the Common Era represented changes of 3–7 % in the modern interannual range of variability in precipitation, but the accumulation of these long-term trends over the entirety of the Holocene caused recent centuries to be significantly wetter, on average, than most of the past 11 000 years.

Climate of the Past↗