Search for: All records

Abstract The termination of the last glacial period is marked by the northward migration of the ITCZ and the weakening of the South American Summer Monsoon (SASM). The transition between the wetter glacial period and the more arid Holocene period across the South American continent is punctuated by several abrupt millennial-scale tropical hydroclimatic events. While the Northern Hemisphere temperature forcing of these millennial-scale events is generally accepted, recently, equatorial forcing mechanisms have been put forward. In particular, the dipole between northeastern Brazil and the western Andes of Peru is absent during Heinrich 1, with wet conditions recorded in both regions. To explain this anomalous atmospheric behavior, researchers have suggested changes in the ENSO and Walker circulation over South America and questioned whether the ‘amount effect’ relationship between δ18O and precipitation persists through time. To better resolve tropical hydroclimate changes over the last glacial termination, more robust paleoclimate proxies are needed. Here, we present a new paleo-precipitation reconstruction based on trace metal (Mg/Ca, Sr/Ca, and Ba/Ca) and isotope (δ18O and δ13C) speleothem records from Antipayarguna cave in the Peruvian Andes (3800 masl). Our records date from 2,600 to 4,700 and 7,700 to 19,000 years BP, with an average age resolution of 44 years. These records overlap the previously published speleothem records from nearby Pacupahuain and Huagapo caves. The Antipayarguna δ18O data are highly correlated with southern hemisphere summer insolation and the Huascaran ice core δ18O record. The Antipayarguna trace metal ratios and δ18O isotope values correlate well over most of the record, suggesting that the δ18O at our site reflects the amount of local precipitation. However, at the end of the Younger Dryas (11.5-10.3 ka) and Heinrich Stadial 1 (16.4-14.9 ka), there is a decoupling of these proxies. These anomalies may be due to changes in δ18O caused by shifts in moisture source region or precipitation condensation factors (e.g. convergence level or subcloud evaporation). Alternatively, this could be due to a change in trace metal sources. We explore potential causes for these brief decoupling events through comparison with other paleoclimate records. 

Adamussium colbecki is a large thin-shelled scallop common in Antarctic waters and well represented in the fossil record. Shell oxygen (δ18Os) and nitrogen isotopes in carbonate bound organic matter (δ15NCBOM) have the potential to record sea ice state over time. To test this hypothesis we will analyze A. colbecki shells from Explorers Cove and Bay of Sails, western McMurdo Sound, Antarctica. These sites have different sea ice states: persistent (multiannual) sea ice at Explorers Cove and annual sea ice (that melts out every year) at Bay of Sails. Six adults shells collected at these sites in 2008 (3 from each site) and two juveniles collected in 2016 from Explorers Cove will be serially sampled for δ18Os values from the growing shell margin to the umbo. We hypothesize that melting glacial ice will pulse freshwater with low δ18O values into the system, which will be recorded in as larger amplitude pulses in shells from the Bay of Sails, but as dampened pulses in Explorers Cove. Carbonate bound organic material will be sampled for δ15NCBOM values. Recent studies illustrated that δ15NCBOM values provide a similar proxy as soft tissue δ15N values (Gillikin et al., 2017, GCA, 200, 55–66, doi: 10.1016/j.gca.2016.12.008). The organic content of the shell is low and the shells are thin, so δ15NCBOM values will be more time averaged than δ18Os values. Nevertheless, sea-ice organic N should have higher δ15N values compared to open water organics due to nitrate draw down in the ice (Fripiat et al., 2014, Global Biogeochem. Cycles, 28, 115–130, doi:10.1002/2013GB004729). Thus we expect large differences between Explorers Cove with persistent sea ice cover and Bay of Sails where the sea ice melts out every year. We posit that oxygen and nitrogen isotopes in A. colbeckishells have a high potential to record sea ice cover. 

The Antarctic scallop Adamussium colbecki may be a crucial paleoenvironmental proxy for coastal Antarctica. For example, two highly seasonal environmental parameters, glacial melt and productivity, were linked to trace elemental concentrations in a previous bulk shell analysis and a transect spanning ~ 3 months of juvenile growth. However, neither study examined seasonal variation in trace elements or tied variation to distances between small ridges (striae) on valve surfaces, which may also vary seasonally. Striae and interstrial growth between them are expressed as alternating narrow and wide groups (presumably winter and summer growth, respectively). If tied to trace elemental concentrations, striae could provide high-resolution sclerochronological proxies for seawater conditions. Here, we evaluate whether trace elements archived in A. colbecki striae can be used as seasonal indicators of glacial influence and nutrients over A. colbecki ontogeny. We examined trace elements from an adult and juvenile A. colbecki (shell height, 80.2 mm and 17.1 mm, respectively) collected live by divers from ~ 12 m water depth in Explorers Cove, western McMurdo Sound (2008 and 2016, respectively). Trace elements linked to glacial melt (Mg/Ca, Mn/Ca, Fe/Ca, and Pb/ Ca), metabolism (Mg/Ca), and productivity (Ba/Ca) were sampled with an LA-ICP-MS on each stria along the central growth axis of lower (right) valves from umbo to growing margin. Distances between sampled striae were measured along the central margin (FIJI). Interstrial distances (ISDs) and trace elements were compared using wavelet coherence analysis (Wavelet- Comp 1.1) and cross-correlation. Coherence and correlations that exceeded 95% significance are reported here. Coherence identifies areas of covariance between ISD and trace elements over ontogeny; cross-correlation describes the direction (±) of correlation between 113 NAPC 2019 PROGRAM & ABSTRACTS ISDs and trace elements where coherence exists. We expected trace elements that increase with glacial melt (Fe, Mn, Pb), productivity (Ba), and altered metabolism (Mg) to be coherent and correlate positively with ISD (highest concentrations at wide summer striae) throughout ontogeny. Preliminary results mostly do not conform to predictions. Though correlation remains consistently positive or negative under strong coherence, most elements are only coherent with ISD for short strial sequences (~ 8 striae) and only during adult growth. Of the elements associated with glacial melt, only Mn correlates positively with ISD and may be a potential proxy for seasonality. Other indicators of glacial melt (Pb/Ca, Fe/Ca) and productivity (Ba/Ca) correlate negatively with ISD. Mg/Ca correlates positively with ISD, indicating seasonal effects on metabolism. Ontogenetic variation in coherence urges cautious use of ISDs as proxies, but Pb/Ca (anthropogenic in Antarctica) is coherent with ISD throughout ontogeny; further analysis might illuminate seasonal effects of human activities on Antarctic ecosystems. 

Sea ice is critical in structuring Antarctic marine ecosystems, controlling disturbance and primary productivity. Sea ice either melts annually or persists for multiple years, but variability in sea-ice duration is poorly understood prior to satellite images. The Antarctic scallop Adamussium colbecki, with its circum-Antarctic distribution and Holocene fossil history, may be a proxy for sea-ice duration. Previous work on A. colbecki links some trace elements to ice melt and productivity. Further, increments between growth bands (striae) are thought to vary seasonally. To evaluate A. colbecki suitability as a sea-ice proxy, we tested correspondence between growth and trace elements known to represent sea ice or productivity at two sites in western McMurdo Sound: Explorers Cove (EC) with multiannual sea ice and Bay of Sails (BOS) with annual sea ice. Trace element signals should be dampened or absent at EC, whereas those from BOS should cycle annually. One A. colbecki shell each from EC and BOS were collected live in 12 m of water. Trace elements previously linked to ice melt (Mn/Ca, Fe/Ca, and Pb/Ca), metabolism (Mg/Ca), and primary productivity (Ba/Ca, Li/Ca) were sampled from interstrial increments using an LA-ICP-MS along the central axis from umbo to last striae. Interstrial distances (ISDs) were measured and compared to trace elements using wavelet coherence analysis. Coherence (covariance between ISD and trace elements) exceeding 95% significance are reported here. Results indicate that ISD and trace elements only cohere during episodic sea-ice melt at EC and cohere throughout adult growth at BOS. All EC trace element concentrations display a common pattern: cyclic growth followed minimal variation in early adult ontogeny, with intermittent variation resuming later in adult growth. In contrast, trace elements from the BOS scallop exhibit strong cyclic behavior throughout ontogeny. ISD coheres with trace elements at EC for short strial sequences (5-30) twice in adult growth, corresponding to partial sea ice melts at EC during 1999 and 2002. Conversely, BOS trace elements cohere with ISD for long (20-140) strial sequences during adult growth, indicating annual sea-ice melt. Results indicate that A. colbecki archives sea-ice duration, thus its fossil record can be used to investigate past variability.