Ten cores, ranging from 18 to 70 cm in length, were obtained in September 1999 in northern California, nine from Medicine Lake and one from Blanche Lake (Figure C3, Table 1). The cores were taken to assess whether Medicine Lake may contain a high-resolution record of the Holocene climate history of the region. The climate history would be inferred through studies of microplankton, pollen and lake & sediment geochemistry.

The most obvious climate cycle that we expect to be present in Medicine Lake sediments is that of lake level. Because Medicine Lake is spring-fed, lake-level variations should correlate with regional precipitation. For example, according to eyewitness accounts, the lake was much smaller during a well-documented period of extended drought during the 1930s. Given the lake's bathymetry, with extensive areas shallower than 10 feet (~ 3m), extended drought would drastically alter the lake area. Preliminary interpretation of the sediments from the shallow-water cores (Cores 2-8) reveals clearly defined cycles of lake level that we hope to date and relate to published climate records. Though the record contained in the shallow-water sediments undoubtedly is interrupted by periods of subaerial exposure or erosion, sediments from the deeper part of the lake (Core 9) are likely to contain an uninterrupted record of lake level. Diatoms and geochemistry have proven to be excellent monitors of lake level, so they will be studied extensively in the deep cores.

This report summarizes the coring methods, lithostratigraphy, texture, and depositional environments identified in the core strata from this inital study.

Summary

Data from the shallow lake cores show that an initial lagoonal/marsh environment was drowned, allowing coarse-grain lake sediments to be deposited. Subsequently, there was a drop in lake level and subaerial soils were developed on the lacustrine strata. A rise of lake level in 1940's(?) again flooded the terrestrial soils and resulted in deposition of the thin modern lake sediments.

Table 1. Information on core locations and characteristics.

Core	Depth (cm)	Length (cm)	North Latiitude	West  Longitude	Coretype	Area	Strat. Column	Textures
1	0	70	41° 35.3'	121° 36.2'	Hammer	Barrier Island north side Medicine Lake	Strat. Column	Texture Column
2	100	69	41° 35.6'	121° 35.6'	Hammer	North side Medicine Lake	Strat. Column	Texture Column
3	130	31	41° 35.2'	121° 36.2'	Hammer	North side Medicine Lake	Strat. Column	Texture Column
4	50	62.5	41° 34.6'	121° 35.6'	Hammer	Lagoon north side Medicine Lake	Strat. Column	Texture Column
5	110	31	41° 34.7'	121° 36.5'	Hammer	South side Medicine Lake	Strat. Column	Texture Column
6	100	18.5	41° 35.6'	121° 35.6'	Boxcore	North side Medicine Lake	Strat. Column	Texture Column
7	100	18	41° 35.6'	121° 35.6'	Boxcore	North side Medicine Lake	Strat. Column	Texture Column
8	30	49.5	41° 33.4'	121° 33.2'	Hammer	North side Blanche Lake	Strat. Column	Texture Column
9	3700	27	41° 34.9'	121° 35.4'	Gravity	East central Medicine Lake	Strat. Column	Texture Column

Depositional environments of Medicine Lake

Six cores were obtained in the northern part of the Medicine Lake, where a narrow coarse-grained barrier island has built to the east into the lake and now separates a broad, shallow, grass-covered lagoon from the open lake (Figure 2). One core (core 5) was taken along the southern margin of the lake, and the gravity core (core 9) was taken in the deeper, eastern part of the lake.

We identify at least six distinct lithostratigraphic sequences in the lake strata that include the following depositional environments: 1.) Modern shallow lake, 2.) Deep lake, 3.) Barrier island, 4.) Lagoonal and barrier island overwash, 5.) Terrestrial strata, and 6.) Lacustrine/marsh to lagoonal sediments.

Modern shallow lake sediments

Modern sediments within the shallow part of the lake are now being reworked. They overlie consolidated sediments with an erosional contact. Discontinuous patches of grass 2 to 3 cm in height may cover part of the shallow lake where actively migrating ripples are not present. The modern lake sediments form a distinctive unit ranging from 2 to 14 cm thick. Coarse-grained sand and gravel make up to 100 percent of the sediment. Silt and clay are generally lacking or constitute less than 3 percent of the sediment in this upper stratigraphic interval. The sand fraction is composed of a mixture of tephra clasts, consisting of red, black, tan to cream colored vesicular shards and clear bubble wall shards.

Deep lake sediments

The soft dark brown deep lake sediments contain the highest percentage of silt and clay (up to 96 percent) with the sand fraction ranging from 3.6 to 32.7 percent (core 9). Rare gravel clasts (possibly ice-rafted) are also found in the soft mud. Most of the sand-size fraction consists of biologic components consisting of diatoms, fern and conifer spore cases, seed pods, plant material, and insect parts.

Barrier island

The barrier island strata consist of 70 cm of repeated graded beds of gravel and sand (core 1). The gravel fraction ranges from 0.4 to 60 percent while the sand fraction ranges from 42.2 to 99.9 percent. The sand and gravel fractions are composed of a mixture of tephra clast compositions. The strata represent repeated winnowing and deposition of coarse-grain sediments by waves and wave runup on the narrow barrier island beach.

Lagoonal and barrier island overwash

The lagoonal strata, 30 cm thick, comprise two distinct depositional elements, a basal laminated gray and brown sandy-silty clay lacking gravel-sized-clasts overlain abruptly by coarse-grained sandy gravels. The silt and clay fraction makes up to 48 percent of the sediment in the basal beds changing up 10 to 16 percent in the overwash strata. The sand (up to 76 percent) and gravel (up to 20 percent) form a mixture of tephra grains and clast types in the upper depositional unit. The strata represent quiet water deposition for the basal strata with barrier-island overwash introducing the gravelly sands into the lagoon.

Terrestrial strata

The hard, overconsolidated, red to orangish brown sandy mud, which underlies the modern lake and lagoonal sediments, contains iron concretions, silt clasts, and black to brown roots to 10 cm length forming a distinctive unit ranging in thickness from 16 to 22 cm. An abrupt increase in the <0.062 mm size fraction, from 38 to 78 percent, characterizes this sequence. The sand fraction ranges from 22 to 56 percent. Gravel-sized tephra clasts are usually lacking. Most of the sand-size shards are altered. Apparent exposure of originally lacustrine and lagoonal/marsh sediments has resulted in soil development (high silt and clay fraction) and alteration of the sediment. The sequence was deposited in a terrestrial environment.

Lacustrine/marsh to lagoonal sediments

The stratigraphically lowest depositional sequence found in cores 2, 3, and 4 contains two distinct units, a lower marsh/lagoonal sequence up to 26 cm thick overlain by an upper thin lacustrine deposit 4 to 13 cm thick. The lacustrine strata consist of "clean" sand (97 to 100 percent) or gravelly sand (4 to 16 percent gravel, 52 to 75 percent sand) composed of a mixture of tephra compositions. The laminated lagoonal/marsh sediments contain 48 to 71 percent silt and clay and 28 to 51 percent sand indicating deposition in a quiet-water environment.

Blanche Lake

Blanche Lake contains a similar depositional history as recorded in the strata in Medicine Lake with a thin, 6 cm thick, sandy gravel representing the modern lake sediments overlying gravelly, sandy mud from a terrestrial environment with soil development. The gravel clasts are scattered throughout the strata with clasts up to 6.2 cm in length.