Carbon Isotope Measurements of Surface Seawater from a Time-Series Site Off Southern California

Abstract

We report carbon isotope abundances of dissolved inorganic carbon (DIC) in surface seawater collected from a time-series site off the Newport Beach Pier in Orange County, California. These data represent the first time series of Δ14C data for a coastal southern California site. From a suite of samples collected daily from 16 October to 11 November 2004, Δ14C values averaged 32.1 ± 4.4‰. Freshwater input from the Santa Ana River to our site caused Δ14C and δ13C values to decrease. Since this initial set of measurements, a time-series site has been maintained from November 2004 to the present. Surface seawater has been collected bimonthly and analyzed for Δ14C, δ13C, salinity, and ΣCO2 concentrations. Water samples from the Santa Ana River were collected during the wet season. California sea mussels and barnacle shells, ranging from 4 to 6 months old, were also collected and analyzed. Results from May 2005 to January 2008 show no long-term changes in δ13C DIC values. Δ14C DIC values over the 2005–2006 period averaged 33.7‰; high Δ14C values were observed sporadically (every 6–7 months), suggesting the presence of open water eddies at our site. Finally, in 2007, a stronger upwelling signal was apparent as indicated by correlations between Δ14C, salinity, and the Bakun index, suggesting that the Δ14C record is an indicator of upwelling in the Southern California Bight.

Δ14C and Δ13C of Seawater DIC as Tracers of Coastal Upwelling: A 5-Year Time Series from Southern California

“…It is interesting to note that the 2 periods of largest  13 C variability were in the spring of 2006 and 2010, both following unusually wet periods in Southern California. Those results are not due to input of river water that usually contains low  13 C of DIC values (Hinger et al 2010). Note that our DIC  13 C and DIC  14 C associated with lower salinity values in Figure 2c are not shown or discussed in Figure 2a,b, and therefore cannot be evoked to explain the variability observed.…”

“…Periods of higher salinity (~33.8 psu compared to an average of ~33.5 psu) can be seen in the spring of 2007 and 2008 at the same time as peaks in upwelling intensity (Figure 2e). The seawater samples associated with the DIC  13 C and DIC  14 C shown in Figure 2(a,b) are screened for salinity, and samples of 33 psu or less are discarded because the geochemistry of these samples are likely to be influenced by freshwater from the nearby Santa Ana River (Hinger et al 2010). …”

“…Non-poisoned seawater samples were measured for salinity at the Scripps Oceanographic Institution Data Facility. Salinity was identified in Hinger et al (2010) as a key characteristic of seawater, which can aid in the identification of upwelling events in the SCB. However, salinity is also useful for screening samples that are influenced by freshwater from the Santa Ana River after rare rain events, which causes lower DIC  13 C and DIC  14 C (Hinger et al 2010).…”

“…Recently, Hinger et al (2010) demonstrated that DIC  14 C in seawater can be correlated with the onset and duration of recent coastal upwelling at a site in the Southern California Bight (SCB), when combined with other proxies, such as salinity and the Bakun upwelling index. This index is an estimate of the volume of seawater upwelled along the coast due to geostrophic winds and is constructed using surface atmospheric pressure fields (Bakun 1973).…”

“…To examine whether the relationships between salinity, DIC  14 C, and upwelling are consistent in the longer term, we continued to study the salinity, temperature, and carbon isotope abundances of DIC in surface seawater collected biweekly at a coastal site off Southern California within the Southern California Bight (SCB) from [2008][2009][2010]. Here, we show the complete time series of carbon isotope abundances of DIC from 2005-2010, including data from Hinger et al (2010). This represents a unique time series of coastal data against which new proxies can be calibrated, so that we can understand the influence of upwelling on coastal seawater geochemistry over longer timescales.…”