Scientists cored Clear Lake, finding ancient pollen records in sediment that reveal how California forests changed with past climate shifts

Clear Lake in Northern California might appear from the outside just like any other lake surrounded by vineyards and towns, yet USGS and UC Berkeley summaries of its sediment cores describe it as one of the oldest lakes in North America, with a basin that has been collecting sediment for up to roughly 500,000 years. It hides, however, an extraordinary feature in its depth: USGS researchers have documented a basin where sediment layers have accumulated for hundreds of thousands of years without ever fully draining or freezing over. This largely continuous sediment sequence made this lake an important location in North America for studies of forest history in California, including the redwood group.

Clear Lake’s basin has preserved sediment for roughly 500,000 years

Clear Lake is found in the fault-bounded basin of the northern California Coast Ranges, which has helped keep it stable despite multiple ice ages that have removed almost all lakes found at comparable latitudes. Field studies from UC Berkeley and the USGS Professional Paper 1363 indicate that Clear Lake's sediments preserve a climate and vegetation record extending back as far as half a million years, with only three notable breaks in continuity. This near-continuous record is unique; most of the lakes found in glaciated environments were cleared of their sediments multiple times.


Early USGS coring reached back 130,000 years

The earliest systematic study of such a record took place in 1973, when Geological Survey field teams collected eight sediment cores from various locations in Clear Lake, a campaign later written up in USGS Open File Report 79‑1085. The longest of these cores, a 115‑meter core from the lake’s main basin (often referred to as Core 4 or CL‑73‑4), was described in USGS reports on the 1973 coring campaign, while a separate 27.4‑meter core from the Highlands Arm (Core 7) was documented in the report Pollen Counts from Core 7, Clear Lake, Lake County, California, authored by David P. Adam.

This study mentions that an accompanying long core covers the period from the end of the second-to-last glacial age around 130,000 years ago to the present day. Later, a follow-up USGS study of a very long core drilled in 1980 (Core CL‑80‑1) also references the 130,000‑year continuous pollen record from the earlier 1973 core as a benchmark for the region’s paleoclimate research.

Core 7's pollen record tracks oak, pine and Redwood-family pollen

A USGS report on Core 7, a 27.4-meter-long sample taken from the Highlands Arm of the lake, shows in detail what the record looks like. The base of the core has been dated by carbon-14 to be over 40,000 years old, while the transition between shallow and deep lake environments occurred approximately 11,300 radiocarbon years ago. Across that span and continuing into the more recent record, three pollen types appeared dominant, according to the same USGS report: oak (Quercus), pine (Pinus) and TCT (Taxodiaceae, Cupressaceae and Taxaceae). Additionally, the USGS report on Core 7 stated that oak pollen increased during warmer periods and decreased during colder ones over the longer record, providing a vegetation-based proxy for past climate swings.

Redwood pollen is part of a broader conifer signal, not a standalone marker

It is important to distinguish what the Clear Lake cores can identify from what they cannot. Redwood pollen has not been tracked separately in the pollen chronology because its grains are difficult to distinguish from cypress, juniper and cedar under a microscope. It follows that any changes in the TCT family would reflect changes across all of the plants in the conifer family.

A 2012 UC Berkeley project sampled the site at a far finer resolution

Decades later, UC Berkeley integrative biology professor Cindy Looy led an effort to drill new cores from the same general region of Clear Lake, near the town of Lucerne, a project UC Berkeley covered in its 2012 news release on the coring campaign. The collaboration involved no fewer than 17 faculty members, ranging from paleontologists to experts in pollen and climate modeling, the university reported.

While the previous USGS cores were sampled at coarse intervals (on the order of once per meter), the Berkeley team sampled at much finer, centimeter-scale resolution, greatly increasing the time resolution of the record, UC Berkeley's coverage of the project noted. The university's news release stated that the explicit goal of the research was to use the detailed history of past climate change to improve predictions of how present-day organisms might respond to future warming.

Why long fossil records matter for forecasting forest change

The record’s value lies less in individual data points than in the long-term trend across climate cycles. Scientists can use the record to examine how forest makeup, including conifers such as redwoods, changed during past temperature shifts. This comparison gives scientists a long-term context for comparing past forest change with climate projections.