NON INVASIVE MULTI PROXY APPROACH TO RECOGNIZE PALEO-COASTAL ENVIRONMENTAL SIGNALS

Coastal lagoons are archives of paleo-coastal environmental signatures. Grain size, micro fossil assemblages and organic carbon content are some of the common proxies used in paleo-coastal environmental studies. This study could successfully recognize paleo-coastal environmental changes and paleo-coastal events using non invasive techniques to measure the physical and chemical properties of lagoonal cores.

Four sediment cores, each five meters in length, were collected along a 1km long transect from a siliciclastic coastal lagoon at Kirinda Sri Lanka. Particle size, magnetic susceptibility, XRF and Diffuse Spectral Reflectance (DSR) were measured in all four cores at 1-2cm resolution. XRD analysis was carried out to recognize clays. R mode principal component analysis (PCA) was carried out on the DSR data.

PCA analysis of the first derivatives of combined reflectance data recognized three principle components (PCA) which describe 96.1% of the total variance. PCA1 represents iron oxides while PCA2 represents a mixture of kaolinite + illite and PCA3 shows plant pigments + silica. Down core variation of grain size and PCA2 (Clay) show the spatial and temporal variation of the lagoonal environment. All four cores have mollusk shells and organic matter abundant sand below a clay layer with less organics and no mollusk shells. A sand layer with less organics overlies the clay layer. This lithostratigraphic sequence suggests a gradual filling of a high energy shallow lagoon with terrestrial sediments as a result of sea level regression. Down core increase of sulfur and Ca levels and up core increase of PCA 1 (iron oxide) and magnetic susceptibility also suggest a transition from a reducing lagoonal environment to an oxidizing terrestrial environment. Variation of PCA3 (plant pigments) shows the variation of productivity in response to lagoonal environmental variation and climatic variations.

According to an age model constructed for a nearby lagoon by Jackson (2009) beginning of thw high energy lagoonal environment may correlate to 6000 yrs BP and represent the mid-Holocene sea level transgression.

Signatures of the 2004 tsunami event and several similar older instant costal events could be recognized using these parameters, particularly in the PCA3 which may represent the ratio of freshwater to marine biomass in the cores.