A few years ago, we were surfing at Cardiff-by-the-Sea over the magical reefs that lend the area incredible surfing conditions and shapely wave formation. Something out of the ordinary caught my eye – paleosols sandwiched in the beach cliff! At the south end of San Elijo State Beach, near the rivermouth – there was a perfectly preserved sequence of three deeply weathered paleosols (ancient, buried soils). Even more exciting was the realization that Cardiff Reef itself (the shore platform extending out to sea) was part of the paleosol sequence! Who could have imagined – surfing atop an ancient soil!
After a short review of the geological history of the region, it was apparent that we were dealing with REALLY OLD soils – between 50 and 55 million years old! Even more exciting was that these soils had the telltale signs of intense, tropical weathering: the profiles were dominated by kaolinite and were very thick, approximately 3 meters deep. So, our surfing/geology side project began. We returned in January of 2021 to describe and sample the ancient soils at three locations where they are naturally exposed in the beach cliffs: San Elijo, Black’s Beach, and Torrey Pines. The objective of this work was to constrain the paleoclimate conditions (mean annual precipitation, mean annual temperature) during soil formation. Critically, this period in geological history (known as the Paleocene-Eocene “thermal maximum” was the last time that atmospheric CO2 was as high as is projected for the year 2100 (between ~1000 and 2000 ppmv CO2, we are currently at ~420 ppmv). Therefore, studying soils of the past can help to constrain how climate and ecosystems will respond to modern global warming, and provide unprecedented insight into the climate evolution of present day southern California during the Paleocene-Eocene thermal maximum.
Abstract: Modern deserts of southern California are known for their minimally weathered soils and low annual precipitation. However, newly described early Eocene (50-55 million-year-old) paleosols (ancient, buried soils) in beach cliffs of northern San Diego County, California reveal severe weathering under a greenhouse climate millions of years ago in present-day California. Here we provide a reconstruction of climate and weathering intensity on land during the early to mid-Eocene in southern California using the geochemistry and morphology of deeply weathered nonmarine sedimentary rocks. Early Eocene (~55 Ma) kaolinitic Oxisol paleosols developed atop a parent material of alluvial quartzite that was subject to intense subaerial alteration (CIA >98), characteristic of severe weathering under subhumid tropical conditions. Less altered Middle Eocene (50 Ma), high shrink-swell (Vertisol) paleosols developed in siliciclastic sediments are also highly weathered (CIA >75), though to a lower degree relative to the older Oxisols, suggesting a decline in weathering intensity over ~5 Ma. Subtidal to supratidal mangrove paleosols also Eocene (~50 Ma) in age contain pyritized root traces and coalified wood which altogether allow for a reconstruction of landscape ecology and climate. We discuss paleoclimate transitions across landscapes and implications for early Eocene climate and weathering in present-day southern California.