3.3. D-excess analysis
Spatial variations of reconstructed D-excess are explained by
theoretical considerations such as the effects of (i) distance or
proximity to main moisture sources to the study areas (Guan et al.,
2013); (ii) influence of oceanic or terrestrial sources (Aemisegger et
al., 2014); (iii) depletion of the heavy isotopes from coastal regions
toward the continental interiors (Gat & Matsui,1991; Rozanski et al.,
1993). Particularly, for the Andean region, the effects of oceanic or
terrestrial sources and the depletion of heavy isotopes with distance to
coastal regions are most evident (Upper left inserts in Fig. 7). This
can be explained by the predominance of terrestrial sources depleted in
δ18O and δ2H and the moisture
transport route (e.g. from the Atlantic Ocean is larger than the
Caribbean region causing major fractionations). For the Caribbean
region, the proximity effect of main moisture sources to the target
region is the most visible, given that, for this region, D-excess
reconstruction shows predominant values close to the Caribbean Sea and
the Atlantic Ocean, which agrees with previous reports from Terzer et
al., (2021).
The seasonal variation of D-excess ranged from 7 to 15 ‰ VSMOW, reaching
its maximum in December/January (Fig. 7a, Fig 7b) and its minimum in
June/July (Fig. 7g, Fig 7h). These changes are explained by air-sea
conditions at distinct water vapor sources with specific D-excess values
(Gat & Gonfiantini, 1981) and modified by the mixing of inland water
vapor from evapotranspiration with the air-mass (Gat, Joel, Bowser, &
Kendall, 1994) and by secondary evaporation (Frits, Drimmie, Frape, &
Oshea, 1987). In continental areas, the D-excess value is strongly
controlled by temperature effects (δ18O temperature
coefficient increases to about 0.5 ‰/°C) (Frits et al., 1987). This
effect is evident in our interpolation with lower D-excess in the
Caribbean region, and with higher temperatures and higher D-excess
compared to the Andean region. In addition to the temperature effect,
and considering the Rayleigh distillation phenomenon,
δ18O values become lower when water vapor rains out at
lower temperatures than oceanic sources (Dansgaard, 1964). There is an
inverse relationship between δ18O and D-excess. As
expected, our data in the Andean region shows this pattern, as its
predominant moisture source is terrestrial, and is located away from the
ocean. In the Andean region, the months that showed the highest values
of D-excess correspond to the season with the low values of rainfall,
with December and January being the months with the highest values of
D-excess (Fig. 7a, Fig 7b), reaching up to 15‰ VSMOW. D-excess reduces
in June and July (Fig. 7g, Fig 7h), due to the effect of secondary
evaporation which can be especially large with small amounts of
precipitation during hot dry months. April and May (the rainy season)
report the lowest concentrations of deuterium excess, with a range
variation between 8‰ VSMOW and 10‰ VSMOW.