Solar forcing of Nile discharge and sapropel S1 formation in the early to middle Holocene eastern Mediterranean

Abstract

We present high-resolution records for oxygen isotopes of the planktic foraminifer Globigerinoides ruber (δ 18 O ruber ) and bulk sediment inorganic geochemistry for Holocene-age sediments from the southeast Mediterranean. Our δ 18 O ruber record appears to be dominated by Nile discharge rather than basin-scale salinity/temperature changes. Nile discharge was enhanced in the early to middle Holocene relative to today. The timing of the long-term maximum in Nile discharge during the early Holocene corresponds to the timing of maximum intensity of the Indian Ocean-influenced Southwest Indian summer monsoon (SIM). This coincidence suggests a major influence of an Indian Ocean moisture source on Nile discharge in the early to middle Holocene, while, presently, the Atlantic Ocean is the main moisture source. Nile discharge was highly variable on multicentennial time scale during the early to middle Holocene, being strongly influenced by variable solar activity. This solar-driven variability is also recorded in contemporaneous SIM records, however, not observed in an Atlantic Ocean-derived West African summer monsoon record from the Holocene. This supports the hypothesis that the Indian Ocean moisture source predominantly controlled Nile discharge at that time. Solar-driven variability in Nile discharge also influenced paleoenvironmental conditions in the eastern Mediterranean. Bulk sediment Ba/Al and V/Al, used as indicators for (export) productivity and redox conditions, respectively, varied both in response to solar forcing on multicentennial time scales. We suggest that changes in Nile discharge on these time scales have been concordant with nutrient inputs to, and shallow ventilation of, the eastern Mediterranean.

Humans as Agents in the Termination of the African Humid Period

“…from grassy to shrub-dominated vegetation is recorded in two sediment cores from the mouth of the Nile River at 8700 and after 8000 years BP, but there is no concurrent rapid reduction in river runoff or precipitation suggesting that monsoon activity was only slowly decreasing at that time (Figures 3B,C; Hennekam et al, 2014;Blanchet et al, 2015). There is consistent agreement in the proxy record that, beginning in northern Africa by ca.…”

“…Such discordance in proxy data, even at scales of hundreds of kilometers, is typical for the terminal AHP. (Blanchet et al, 2015); (C) Nile River hydrology and (Continued) FIGURE 3 | Continued vegetation reconstruction (Hennekam et al, 2014); (D) Lake Yoa fine sand flux (dune activation) and pollen spectra (Kröpelin et al, 2008); (E) Lake Tilla pollen spectra in circles (Salzmann et al, 2002); (F) Black dashed line represents the Lake Chad level reconstruction. Elongated ovals are elevation-measured radiometric ages and associated statistical uncertainty (Armitage et al, 2015); (G) Ocean sediment core reflecting eolian activity near Cap Blanc, Mauretania (deMenocal et al, 2000).…”

Hydrological variations of the intermediate water masses of the western Mediterranean Sea during the past 20 ka inferred from neodymium isotopic composition in foraminifera and cold-water corals

“…Similarly, enhanced Nile discharge at ∼ 9.5 cal kyr BP was inferred based on δ 18 O in planktonic foraminifera from a sediment core in the southeast Levantine Basin (PS009PC; 32 • 07.7 N, 34 • 24.4 E; 552 m water depth) (Hennekam et al, 2014). This increasing contribution of the Nile River to the eastern Mediterranean basin has been related to the African Humid Period (14.8-5.5 ka BP; Shanahan et al, 2015), which in turn was linked to the precessional increase in Northern Hemisphere insolation during low eccentricity (deMenocal et al, 2000;Barker et al, 2004;Garcin et al, 2009).…”

Eastern Mediterranean Sea circulation inferred from the conditions of S1 sapropel deposition

“…In addition to orbitally driven insolation changes in a precession cycle, Mediterranean Sea bottom-water oxygenation records indicate centennial to millennial variability (AbuZied et al, 2008;Casford et al, 2003;De Rijk et al, 1999;Hennekam et al, 2014;Kuhnt et al, 2007;Rohling et al, 1997;Schmiedl et al, 2010). One of the most prominent changes occurred around 8 cal ka BP, mainly affecting shallow water masses (Rohling et al, 2015) at water depths of ≤ 1500 m in the southern Aegean Sea (example sites SL123 and LC-21, Fig.…”

“…1a and b) and ≤ 1200 m in the Adriatic Sea (example site MD90-917, not shown in figure; . Both internal processes, such as winter cooling in the northern high latitudes (Schmiedl et al, 2010), and solar activity (Hennekam et al, 2014;Rohling et al, 2002) have been proposed to produce centennial to millennial variability in the eastern Mediterranean Sea.…”