The curse of physiology - challenges and opportunities in the interpretation of geochemical data from mollusk shells

Physiology corrupts geochemical records of mollusk shells in many ways, e.g., by actively controlling the incorporation of trace elements in the skeleton. However, the effects of variable biomineralization rates and growth cessation have largely remained unconsidered. Mediated by endogenous timekeeping mechanisms, mollusks stop growing skeletal material on a regular basis ranging from ultradian to annual timescales. During growth cessation, the shells do not record environmental conditions. Shell growth also stops when environmental conditions are beyond the physiological tolerance of the organism, e.g., above and below genetically determined, species-specific thermal extremes where shell growth slows and eventually ceases. Such growth disruptions can occur at non-periodic time intervals. Due to growth retardations and halts, proxy records of mollusk shells are thus incomplete, and reconstructed environmental amplitudes prone to truncation. Furthermore, environmental records are biased toward the physiological optimum of the animal. Favorable environmental conditions increase shell growth, whereas adverse environmental conditions result in reduced shell production and lowered overall metabolism. Not least, the duration of the growing season and overall growth rate decrease as the mollusk grows older. Mathematical modeling approaches can significantly improve proxy records obtained from mollusk shells. For example, if the duration of growth cessation is known, it may be possible to model the missing environmental record. It is also fairly easy to account for age-related growth trends, or variable time-averaging in different portions of the shell. However, a major premise for a reliable interpretation of proxy records from a mollusk shell or other organisms secreting biogenic hard parts is a proper understanding of the physiology, and of course, a high-resolution record of the many different environmental factors that may influence physiology and shell growth. The present paper reviews examples from the literature, and unpublished data on how physiology influences geochemical proxy records from mollusk shells, and presents methods how to eliminate such adverse effects

A 'clam-ring' master-chronology constructed from a short-lived bivalve mollusc from the northern Gulf of California, USA

Age-detrended and standardized annual growth increment width time series of 67 live-collected intertidal bivalve molluscs of the species Chione cortezi, C. fluctifraga and C. californiensis from the northern Gulf of California were strung together to form a master-chronology that covers the period from AD 1982 to 1999. A high positive correlation was found between standardized annual growth rates and summer sea-surface temperatures and river-flow volume in the period 1988-99; and 63-76% of the variation in mean relative annual growth rates of Chione spp. is explained by temperature and river flow. Seven dead-collected specimens were cross-dated with the master-chronology permitting the reconstruction of the year of hatching and death for these shells. The incorporation of dead-collected specimens in the master-chronology improved the correlation statistics slightly. Species-related growth differences were not significant, but geographic differences in growth rates occur: individuals at the Colorado River mouth grew more slowly when large amounts of fresh water reached the Gulf. This study demonstrates that even short-lived molluscs (average 6-10 years old) can be used to build a master-chronology of environmental and climatic history

Seasonality in the north sea during the allerod and late medieval climate optimum using bivalve sclerochronology

Seasonal temperature patterns may have changed through time in response to current global warming. However, the temporal resolution of available proxy records is not sufficient to quantify paleotemperature seasonality prior to anthropogenic forcing of the climate. In the present study, we reconstructed seasonal and inter-annual temperature patterns of the North Sea during the last 140 years, the Allerod Interglacial and the Late Medieval Climate Optimum using sclerochronological and delta O-18(aragonite) data from bivalve shells, Arctica islandica. On average, the climate during 1278-1353 AD was ca. 1.1A degrees C colder and seasonality was ca. 60% less than today. During the Allerod, long-term temperatures remained about 3.2A degrees C below present values, and absolute summer and winter anomalies were ca. -4 and -2.7A degrees C, respectively. However, seasonality was statistically indistinguishable from today. Long-term average temperatures compare well with existing data for the Late Medieval and Allerod, but detailed information on seasonality during the studied time intervals has never been presented before. Our data also demonstrated that annual instrumental and delta O-18(aragonite)-derived temperatures did not always match. This difference is explained by (1) NAO-driven salinity changes, which influence the temperature estimation from delta O-18(aragonite) and (2) food-driven changes in growth rates; portions of the shell that formed more rapidly are overrepresented in carbonate samples. Our study indicated that individual bivalve shells can open discrete, near-century long, ultra-high-resolution windows into the climate past. Such information can be vital for testing and verifying numerical climate models

Growth increments and stable isotope variation in shells of the Deep-sea hydrothermal vent bivalve mollusk Bathymodiolus brevior from the North Fiji Basin, Pacific Ocean

Bathymodiolus brevior [von Cosel, R., Metivier, B., Hashimoto, J., 1994. Three new species of Bathymodiolus (Bivalvia: Mytilidae) from hydrothermal vents in the Lau Basin and the North Fiji Basin, western Pacific, and the Snake Pit Area, mid-Atlantic ridge. Veliger 37, 374-392] a bivalve mollusk living at deep-sea hydrothermal vents, exhibits daily microgrowth structures in its shell. This interpretation is substantiated by various lines of evidence: (1) similar shell portions of contemporaneous specimens from the same locality contain almost the same number of microincrements; (2) the number of microincrements coincides with the expected number of days in which shell portions of Bathymodiolus spp. form; (3) the width of such microincrements compares well with daily growth rates estimated for the close relative B. thermophilus [Kenk, V.C., Wilson, B.R., 1985. A new mussel (Bivalvia, Mytilidae) from hydrothermal vents in the Galapagos rift-zone. Malacologia 26, 253-271]; (4) different specimens from the same site show similar microgrowth curves. In addition, we found support for tide-controlled shell growth. Daily shell growth rates fluctuate on a fortnightly basis. Some shell portions also revealed the typical tide-controlled microgrowth pattern commonly observed in intertidal bivalves. Based on the analyses of lunar daily growth increments, a growth curve for B. brevior was computed: X-t = 14 cm-(14-0.04 cm) e(-0.26t). This curve enables estimation of ontogenetic age from shell length. According to this equation, B. brevior reaches its maximum shell length of 14 cm at about age 18. Shell isotope analyses suggest that some major shell growth interruptions or retardations are related to extremely active hydrothermal venting activity. However, shell growth also stopped during periods of low venting implying physiological controls on shell formation. Results of the present study demonstrate that shells of B. brevior provide calendars and environmental data loggers that can complement or partly substitute for long-term observations of venting systems

Use of HMDS (hexamethyldisilazane) to dry organic microstructures in etched bivalve mollusk and barnacle shells

The organic framework of molluscan and barnacle shells yields clues to biocalcification processes. Slight demineralization of the shells reveals the fragile meshwork of insoluble organic fibers and membranes, which tend to collapse, wrinkle, and shrink when air-dried from water. Comparison of different drying techniques on etched bivalve mollusk (Chione fluctifraga) and barnacle shells (Chthamalus sp.) reveals that hexamethyldisilazane (HMDS,) produced results qualitatively superior to critical point drying or drying from ethyl alcohol or water. HMDS dries structural details of the organic meshwork excellently and facilitates the recognition of faint growth increments for growth pattern analysis (sclerochronology). The HMDS method is cost-effective, saves time and can be used as a routine substitute for drying microstructures in slightly etched molluscan and barnacle shells