Fig 1.
Map of study region in southern California for illustrative purposes only.
Collection sites (red dots) are indicated at northern Channel Islands (San Miguel Island, Santa Rosa Island, Santa Cruz Island, and Anacapa Island), Newport Beach, and San Diego. Inset map of northern Channel Islands modified from USGS National Map Viewer.
Table 1.
Summary of the 13 papers from which data are sourced for this study.
Table 2.
Treatment of δ18Ow values for this study according to binned age range (years BP).
Fig 2.
Stacked plot comparing seawater temperatures and the longest δ18Oshell profile.
(A) Seawater temperatures for 2000–2005. Black line is a two-week running mean for NDBC Station 46054 in Santa Barbara Basin. Gray open circles are daily SST. Blue line is two-week running mean for Point Dume provided by the Shore Stations Program sponsored at Scripps Institution of Oceanography. Blue open circles are daily seawater temperatures. Numbers indicate maxima for each year of the record. (B) 118 mm of δ18Oshell from a M. californianus individual collected in 2005 at San Miguel Island. Gray dashed line represents the midpoint value of 0.49‰. Labeled numbers represent inferred warm seasons best matched with panel (A). (C) Local polynomial regression of δ18Oshell showing that modeled seasonal cycles become less apparent as the shell ages ontogenetically due to slower growth. Shaded region represents 95% confidence interval. (D) Calculated seasonal amplitude of δ18Oshell (dark circles and dashed line) and annual standard deviation of δ18Oshell (black triangles and solid line).
Fig 3.
δ13Cshell profile of San Miguel Island M. californianus individual collected in 2005.
X-axis represents ontogenetic growth from left to right (e.g., 0 mm is closest to time of death). Gray dashed line represents a median δ13Cshell value of -0.47‰. Regression line (in red) indicates ontogenetic trend towards positive δ13Cshell values.
Fig 4.
Box plots showing stable isotopic ranges of all modern shells by location.
Channel Islands includes all data points from all modern shells with five or more subsamples from San Miguel Island and Santa Cruz Island. (There were no modern shells with more than five subsamples from Anacapa or Santa Rosa Island.) Mainland California includes all data points from all modern shells with five or more subsamples from San Diego and Newport Beach. Black bar is the median value. (A) Oxygen isotope range of modern δ18Oshell in Channel Islands vs. mainland shells. (B) Carbon isotope range of modern δ13Cshell in Channel Islands vs. mainland shells. Arrow shows expected directionality of a potential δ13C upwelling signal.
Fig 5.
Box plots showing stable isotope ranges of each site comprising all shells with five or more subsamples from all Holocene intervals.
Black bar is the median value. (A) Oxygen isotope data from all shells with five or more subsamples for San Miguel Island (SMI), Santa Rosa Island (SRI), Santa Cruz Island (SCI), Anacapa Island (ANI), Newport Beach (NB), and San Diego (SD). Y-axis is inverted to match directionality of temperature proxy as indicated by SST arrow. (B) Carbon isotope data from all shells with five or more subsamples for the same sites. Arrow shows expected directionality of a potential δ13C upwelling signal.
Table 3.
Summary statistics for all shells with five or more subsamples, sorted by site.
Fig 6.
Millennial scale δ18Oshell-inferred temperature variability for each island.
Data were binned by millennium for each island to produce a range of temperature snapshots. Black bar is the median value.
Table 4.
Summary statistics for all shells with five or more subsamples, sorted by both island and millennium.
Fig 7.
Individual oxygen isotope profiles of all 28 long profiles (15+ subsamples) from Channel Islands shells.
Each profile is from a unique shell with collection year or calibrated age in years before present (BP) labeled at the top of the plot. Dashed line represents the mean oxygen isotope value recorded by each individual shell. Red shaded bars are estimations of warm seasons. Y-axis is inverted and the scale is uniform for all plots.
Fig 8.
The last 50 mm of δ18O and δ13C profiles in a M. californianus shell collected from Santa Miguel Island in 2005 CE.
In the top plot (δ18O), y-axis is inverted to match directionality of temperature proxy as indicated by SST arrow. Gray bars in both plots denote δ18O-inferred warm seasons. In the lower plot, arrow shows expected directionality of a potential δ13C upwelling signal.
Fig 9.
All data points from all Channel Islands shells with five or more subsamples binned by time interval.
(A) Each time bin on the x-axis represents all samples within that millennium (e.g., 1000 represents all non-modern shells from 0–1000 BP, etc.). Gray points denote median δ18Oshell value of each time bin. Error bars represent three times the standard error (3SE) from the median for visibility. (B) All data points from Channel Islands shells in oxygen and carbon isotope space over the Holocene. Centroids (medians) are plotted for each sub-epoch. Both horizontal and vertical error bars represent 3SE from the median. Arrow shows expected directionality of a potential δ13C upwelling signal. Seven outliers with values greater than 2‰ or less than -2‰ were omitted from the plot.
Fig 10.
Age versus climate proxy data in calibrated years before present.
(A) Standardized percent clay as a precipitation proxy from Silver Lake in the Central Mojave, California (Kirby et al., 2015) [110]. (B) Percentage of warm species (Globigerinoides ruber in orange, Globoturborotalita rubescens in red, and Neogloboquadrina dutertrei in yellow) from Santa Barbara Basin ODP Site 893 (Fisler and Hendy, 2008) [111]. (C) Ice-volume corrected oxygen isotope SST record from Globigerina bulloides planktic foraminifera at ODP Site 893 A/B (Kennett et al., 2007) [105]. Y-axis is inverted. Dashed line represents linear trend. (D) Carbon isotope record from M. californianus shells analyzed in this study with 5 or more subsamples with the median for each year of the record plotted. (E) Oxygen isotope record from M. californianus shells analyzed in this study with five or more subsamples with median for each year of the record plotted in blue. Y-axis is inverted. Dashed line represents linear trend. Large gray points represent 1000-year median δ18Oshell values as in Fig 9. Blue stars on x-axis denote timing of flood events identified from sediment cores in Santa Barbara Basin by Du et al. (2018) [112]. Shaded red bars denote warm periods and shaded gray bar denotes variable conditions inferred by Kennett et al. (2007) [105].
Table 5.
Identified factors that may impact environmental interpretations from M. californianus stable isotope data.